onnxruntime

Address OOM Issue when exporting Whisper (#15880)

### Description
Remove attention_mask from unnecessary code paths in the whisper export
process.

### Motivation and Context
Current export script frequently hits OOM error when export
whisper-large. Memory profiling shows that this is a result of
generating dummy inputs for the `encoder_attention_mask` input for a
model pass during exporting - in whisper-large, this dummy tensor can be
around 20GB in size.

`encoder_attention_mask` is ultimately a dummy input - it's just there
to satisfy certain BeamSearch requirements. Thus, we're currently
creating a 20GB tensor and passing it to the model, which then discards
the input anyways. By removing the code path to generate a dummy
encoder_mask tensor, we can reduce the memory requirements to export
whisper substantially, while keeping the BeamSearch checks satisfied.

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Co-authored-by: Peter McAughan <petermca@microsoft.com>