[Whisper] Fix decoder position IDs for left-padded batches in longform generation (#48028)

* [Whisper] Fix decoder_position_ids for left-padded batches in longform generation

PR #44130 refactored prepare_inputs_for_generation to stop using cache_position for
input slicing, but inadvertently removed the cumsum-based decoder_position_ids
computation from decoder_attention_mask. Without it, WhisperDecoder falls back to
past_kv.get_seq_length() for all batch items — a single value that ignores per-item
padding — giving wrong position embeddings for left-padded sequences.

With condition_on_prev_tokens=True and heterogeneous batch sizes, shorter sequences
get left-padded, so each item has a different real token length. The restored cumsum
correctly computes per-item positions from the decoder_attention_mask regardless of
padding.

Fixes test_whisper_longform_multi_batch_hard_prev_cond and
test_whisper_longform_no_speech_detection (both failing since Feb 2026).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* [Whisper] Re-enable test_longform_generate_multi_batch_cond_prev

This test was skipped in #44130 with "to be checked asap". It now passes
with the decoder_position_ids fix in the previous commit.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* [Whisper] Move position_ids fix to Whisper-specific prepare_inputs_for_generation

Instead of patching the generic prepare_inputs_for_generation in generation/utils.py,
override it in WhisperForConditionalGeneration to compute decoder_position_ids from
decoder_attention_mask for left-padded batches in longform generation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* format

---------

Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>