[do-not-merge] Illustration how to run audio+text dataloading

[pzelasko]

What does this PR do ?

The key config options to set are:

 +model.train_ds.input_cfg=/path/to/speech_text_input.yaml \
 +model.train_ds.use_multimodal_sampling=true \
 +model.train_ds.batch_tokens=65536 \
 +model.train_ds.token_equivalent_duration=0.08 \
 +model.train_ds.quadratic_factor=4096 \
 +model.train_ds.tarred_audio_filepaths=null \
 model.train_ds.manifest_filepath=null \
 model.train_ds.batch_size=null \
 model.train_ds.batch_duration=null \
 model.train_ds.use_lhotse=true \
 model.train_ds.use_bucketing=true \
 model.train_ds.num_buckets=30 \

The values of batch_tokens, token_equivalent_duration, and quadratic_factor need tuning.

where contents of speech_text_input.yaml are:

 - type: group
    weight: 1.0
    input_cfg:
    - type: nemo_tarred
      manifest_filepath: /path/to/manifest__OP_0..127_CL_.json
      tarred_audio_filepaths: /path/to/audio__OP_0..127_CL_.tar
      tags:
        pnc: 'no'
        source_lang: en
        target_lang: en
        taskname: asr
      weight: 1.0
    - type: nemo_sft_jsonl
      paths: /data/nemo_megatron_sft_chat_qa_data.shuf.jsonl
      language: en
      weight: 1.0

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