Bug when generating confidence scores with timestamps for a buffered rnnt model
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**Describe the bug**

In trying to generate confidence scores with timestamps using an RNN transducer model (`stt_en_conformer_transducer_large`), with buffering, there is a type mismatch error on [this line](https://github.com/NVIDIA/NeMo/blob/9abd81bee41b44a3946a6e535025752eaa4314cd/nemo/collections/asr/parts/submodules/rnnt_decoding.py#L687) `for ts, te in zip(hyp.timestep, hyp.timestep[1:] + [len(hyp.frame_confidence)])`

```
Sample:: 100%|██████████| 1/1 [00:00<00:00, 11618.57it/s]
[NeMo W 2024-12-03 14:55:42 rnnt_decoding:1184] Specified segment seperators are not in supported punctuation {"'"}. If the seperators are not punctuation marks, ignore this warning. Otherwise, specify 'segment_gap_threshold' parameter in decoding config to form segments.
<class 'dict'>
Backend macosx is interactive backend. Turning interactive mode on.
Error executing job with overrides: ['model_path=null', 'pretrained_name=stt_en_conformer_transducer_large', 'audio_dir=/Users/aanchan/work/podcast_transcription_using_nemo/test', 'output_filename=/Users/aanchan/work/podcast_transcription_using_nemo/test_rnn_t_f1.json', 'total_buffer_in_secs=4.0', 'chunk_len_in_secs=1.6', 'model_stride=4', 'batch_size=32', 'merge_algo=lcs', 'lcs_alignment_dir=$PWD/lcs']
Traceback (most recent call last):
  File "/Users/aanchan/work/podcast_transcription_using_nemo/env_nemo_1/lib/python3.10/site-packages/hydra/_internal/utils.py", line 394, in _run_hydra
    _run_app(
  File "/Users/aanchan/work/podcast_transcription_using_nemo/env_nemo_1/lib/python3.10/site-packages/hydra/_internal/utils.py", line 457, in _run_app
    run_and_report(
  File "/Users/aanchan/work/podcast_transcription_using_nemo/env_nemo_1/lib/python3.10/site-packages/hydra/_internal/utils.py", line 223, in run_and_report
    raise ex
  File "/Users/aanchan/work/podcast_transcription_using_nemo/env_nemo_1/lib/python3.10/site-packages/hydra/_internal/utils.py", line 220, in run_and_report
    return func()
  File "/Users/aanchan/work/podcast_transcription_using_nemo/env_nemo_1/lib/python3.10/site-packages/hydra/_internal/utils.py", line 458, in <lambda>
    lambda: hydra.run(
  File "/Users/aanchan/work/podcast_transcription_using_nemo/env_nemo_1/lib/python3.10/site-packages/hydra/_internal/hydra.py", line 132, in run
    _ = ret.return_value
  File "/Users/aanchan/work/podcast_transcription_using_nemo/env_nemo_1/lib/python3.10/site-packages/hydra/core/utils.py", line 260, in return_value
    raise self._return_value
  File "/Users/aanchan/work/podcast_transcription_using_nemo/env_nemo_1/lib/python3.10/site-packages/hydra/core/utils.py", line 186, in run_job
    ret.return_value = task_function(task_cfg)
  File "/Users/aanchan/work/podcast_transcription_using_nemo/rnnt_timestamps.py", line 301, in main
    hyps = get_buffered_pred_feat_rnnt(
  File "/Users/aanchan/work/podcast_transcription_using_nemo/env_nemo_1/lib/python3.10/site-packages/nemo/collections/asr/parts/utils/transcribe_utils.py", line 95, in get_buffered_pred_feat_rnnt
    hyp_list = asr.transcribe(tokens_per_chunk, delay)
  File "/Users/aanchan/work/podcast_transcription_using_nemo/env_nemo_1/lib/python3.10/site-packages/nemo/collections/asr/parts/utils/streaming_utils.py", line 1309, in transcribe
    self.infer_logits()
  File "/Users/aanchan/work/podcast_transcription_using_nemo/env_nemo_1/lib/python3.10/site-packages/torch/utils/_contextlib.py", line 116, in decorate_context
    return func(*args, **kwargs)
  File "/Users/aanchan/work/podcast_transcription_using_nemo/env_nemo_1/lib/python3.10/site-packages/nemo/collections/asr/parts/utils/streaming_utils.py", line 1081, in infer_logits
    self._get_batch_preds()
  File "/Users/aanchan/work/podcast_transcription_using_nemo/env_nemo_1/lib/python3.10/site-packages/torch/utils/_contextlib.py", line 116, in decorate_context
    return func(*args, **kwargs)
  File "/Users/aanchan/work/podcast_transcription_using_nemo/env_nemo_1/lib/python3.10/site-packages/nemo/collections/asr/parts/utils/streaming_utils.py", line 1148, in _get_batch_preds
    best_hyp, _ = self.asr_model.decoding.rnnt_decoder_predictions_tensor(
  File "/Users/aanchan/work/podcast_transcription_using_nemo/env_nemo_1/lib/python3.10/site-packages/nemo/collections/asr/parts/submodules/rnnt_decoding.py", line 569, in rnnt_decoder_predictions_tensor
    hypotheses = self.compute_confidence(hypotheses)
  File "/Users/aanchan/work/podcast_transcription_using_nemo/env_nemo_1/lib/python3.10/site-packages/nemo/collections/asr/parts/submodules/rnnt_decoding.py", line 688, in compute_confidence
    for ts, te in zip(hyp.timestep, hyp.timestep[1:] + [len(hyp.frame_confidence)]):
TypeError: unhashable type: 'slice'
```

A clear and concise description of what the bug is.

From the debugger it looks like hyp.timestamp is a dict, and really the zip should be happening over hyp.timestamp['timestamp'] which happens to be a PyTorch tensor. The slicing over a dict type seems incorrect e.g. hyp.timestamp[1:]

![Image](https://github.com/user-attachments/assets/ac5bee79-e5fc-4d4f-9135-7d09d8c5d30f)

**Steps/Code to reproduce bug**

Please list *minimal* steps or code snippet for us to be able to reproduce the bug.

A  helpful guide on on how to craft a minimal bug report  http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports. 

- Start with the code example [speech_to_text_buffered_infer_rnnt.py](https://github.com/NVIDIA/NeMo/blob/main/examples/asr/asr_chunked_inference/rnnt/speech_to_text_buffered_infer_rnnt.py)
- Add a ConfidenceConfig as in the ASR with [confidence estimation tutorial ](https://github.com/NVIDIA/NeMo/blob/main/tutorials/asr/ASR_Confidence_Estimation.ipynb)
```
from nemo.collections.asr.parts.utils.asr_confidence_utils import (
    ConfidenceConfig,
    ConfidenceConstants,
    ConfidenceMethodConfig,
    ConfidenceMethodConstants,
)

confidence_cfg = ConfidenceConfig(
    preserve_frame_confidence=True, # Internally set to true if preserve_token_confidence == True
    # or preserve_word_confidence == True
    preserve_token_confidence=True, # Internally set to true if preserve_word_confidence == True
    preserve_word_confidence=True,
    aggregation="prod", # How to aggregate frame scores to token scores and token scores to word scores
    exclude_blank=False, # If true, only non-blank emissions contribute to confidence scores
    tdt_include_duration=False, # If true, calculate duration confidence for the TDT models
    method_cfg=ConfidenceMethodConfig( # Config for per-frame scores calculation (before aggregation)
        name="max_prob", # Or "entropy" (default), which usually works better
        entropy_type="gibbs", # Used only for name == "entropy". Recommended: "tsallis" (default) or "renyi"
        alpha=0.5, # Low values (<1) increase sensitivity, high values decrease sensitivity
        entropy_norm="lin" # How to normalize (map to [0,1]) entropy. Default: "exp"
    )
)

```

- Change the decoding strategy and attach the confidence config to the RNNTDecoderConfig being used
```
asr_model.change_decoding_strategy(
        RNNTDecodingConfig(compute_timestamps=True,
                           preserve_alignments=True,
                           confidence_cfg=confidence_cfg)
    )

```

- Run the script with the following arguments

```speech_to_text_buffered_infer_rnnt.py
model_path=null
pretrained_name=stt_en_conformer_transducer_large
audio_dir=/Users/aanchan/work/podcast_transcription_using_nemo/test
output_filename=/Users/aanchan/work/podcast_transcription_using_nemo/test_rnn_t_f1.json
total_buffer_in_secs=4.0
chunk_len_in_secs=1.6
model_stride=4
batch_size=32
merge_algo="lcs"
lcs_alignment_dir=$PWD/lcs
```

An example code file and input is in [this Google Drive folder](https://drive.google.com/drive/folders/1fzYkRT6ZRQcU7BvJLTrNCyB4bYWbN8uB?usp=sharing) 

**Expected behavior**

A clear and concise description of what you expected to happen.

The expected output was a json file with time stamps written out.

**Environment overview (please complete the following information)**

 - Environment location: [Bare-metal, Docker, Cloud(specify cloud provider - AWS, Azure, GCP, Collab)] 
 The environment is a local laptop installation
 - Method of NeMo install: [pip install or from source]. Please specify exact commands you used to install.
 
```
pip install Cython packaging
pip install --upgrade pip
export BRANCH="main"
pip install torch torchvision torchaudio 
pip install git+https://github.com/NVIDIA/NeMo.git@$BRANCH#egg=nemo_toolkit[asr]

```

 - If method of install is [Docker], provide `docker pull` & `docker run` commands used

**Environment details**

If NVIDIA docker image is used you don't need to specify these.
Otherwise, please provide:
- OS version : MacOS Monterey 12.5 
- PyTorch version : 2.5.1
- Python version 3.10.4

**Additional context**

Add any other context about the problem here.
Example: GPU model

This was run on a CPU, and not a GPU
