Fix ONNX save crash when ModelProto.ByteSize() raises EncodeError

[anujj]

Summary

This PR fixes a crash in ONNX model saving for large/complex models where model.ByteSize() can raise a protobuf EncodeError (Failed to serialize proto).

Problem

In model_proto_to_file, we probe model size with model.ByteSize() when save_as_external_data=False.
For some models, that call fails before save, causing the pass to crash.

Root Cause

ByteSize() can trigger protobuf serialization internals and fail with:
google.protobuf.message.EncodeError: Failed to serialize proto.

Fix

• Wrap model.ByteSize() in try/except.
• If size probing fails, log a warning and force save_as_external_data=True.
• Preserve existing large-model fallback behavior (<= 0 or >= onnx.checker.MAXIMUM_PROTOBUF).

Validation

• Reproduced failure before fix in mq:matmulnbitstoqdq flow.
• Re-ran with fix and workflow completed without the previous ByteSize() crash.

Impact

• Improves robustness for large ONNX exports.
• No behavior change for normal models where ByteSize() works.

[anujj]

@jambayk for review

[anskumar01]

Also adding @devang-ml