Option to control matmul precision in transcribe speech
jenkins
# What does this PR do ?

We're adding an option to control matmul precision in transcribe speech and chunked infer scripts.
By setting it to `medium` or `high` we achieve **about 2x speedup** in transcription with Canary, without impact on WER (to be confirmed on larger scale tests). Possibly other models can benefit from that setting as well, esp. if they were trained with bfloat16.

**Collection**: ASR

# Changelog 
- Option to control matmul precision in transcribe speech

# Usage

```bash
python NeMo/examples/asr/transcribe_speech.py \
  dataset_manifest=input.json \
  output_filename=output.json \
  batch_size=32 \
  pretrained_name=nvidia/canary-1b \
  gt_text_attr_name=answer \
  matmul_precision=medium
```

# Jenkins CI
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