Accept None as an argument to decoder_lengths in GreedyBatchedCTCInfer::forward

[galv]

GreedyCTCInfer::forward already allowed for this, so they did not implement the exact same interface. Now, they do.

Also warn about not passing in the decoder_lengths argument. It is likely an error on the user's part not to pass it in explicitly.

What does this PR do ?

Accept None as an argument to decoder_lengths in GreedyBatchedCTCInfer::forward

Collection: ASR

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