remove stacking operation from batched functions

[lilithgrigoryan]

What does this PR do ?

PR removed torch.stack calls from batched function yielding to faster decoding

Collection: ASR

Changelog

Changes in nemo/collections/asr/modules/rnnt.py

Refactoring

• batch_initialize_states() renamed to batch_stack_states
  • Reason: The function does not initialize any states, so the name was changed for clarity.
• batch_initialize_states() minimized torch stack calls
  • Reason: Torch stack internally copies the data.
• batch_select_state()
  • Reason: Function is hard to read, tried to make it clearer.

Argument Removal

• batch_initialize_states(): Removed the batch_states argument as it was unused.
• batch_score_hypothesis(): Removed the batch_states argument as it was unused.

Removing redundant calculations

• batch_score_hypothesis(): removed torch.stack calls since they are not needed. Returning python lists instead.
• batch_score_hypothesis(): removed lm_tokens return value as it was not used.

Changes in [nemo/collections/asr/modules/rnnt_abstract.py]

• reflected comments and argument list changes in derived classes here

Changes in [nemo/collections/asr/parts/submodules/rnnt_beam_decoding.py]

• replaced batch_select_state() calls with index access as now everything is a python list
• recombine_hypotheses() returning final hypotheses instead of initial ones.

Usage

• You can potentially add a usage example below

# Add a code snippet demonstrating how to use this 

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