[Fix] ConformerEncoder forward when length is None
Signed-off-by: Ante Jukić <ajukic@nvidia.com>

# What does this PR do ?

This PR fixes a bug in `forward` of `ConformerEncoder` when `length is None`

**Collection**: ASR

# Changelog 
- Fixed initialization of `length` with `new_full`

# Usage

Currently, the following
```
import torch
from nemo.collections.asr.modules.conformer_encoder import ConformerEncoder

# create layer
conformer_encoder = ConformerEncoder(feat_in=10, n_layers=1, d_model=16)

# run
out, out_length = conformer_encoder(audio_signal=torch.rand(10, 9, 32), length=None)
```

results in a `TypeError`:

```
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "${PATH}/miniforge3/envs/nemo_env/lib/python3.9/site-packages/torch/nn/modules/module.py", line 1482, in _call_impl
    return forward_call(*args, **kwargs)
  File "${NEMO}/nemo/core/classes/common.py", line 1087, in __call__
    outputs = wrapped(*args, **kwargs)
  File "${NEMO}/nemo/collections/asr/modules/conformer_encoder.py", line 416, in forward
    length = audio_signal.new_full(
TypeError: new_full(): argument 'size' (position 1) must be tuple of ints, not int
```

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- [x] Bugfix
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# Additional Information
* Related to # (issue)
