Petermca/whisper dedup (#15365)
### Description
Apply `get_shared_initializers()` to the encoder and decoder subgraphs
of Whisper before chaining and exporting the full, final model.
### Motivation and Context
The Whisper export process has some overlap between the encoder and
decoder subgraphs due to the format of the BeamSearch contrib op.
Consequently, there is some shared model data that is duplicated in the
final exported product, which can result in a file size increase of
~40%. This PR takes the methods in `convert_generation.py` and applies
them during the whisper export process.
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Co-authored-by: Peter McAughan <petermca@microsoft.com>
