⚡ Bolt: Optimize rebuild_padding with lazy platform resolution

[ZeyuChen]

Motivation

The rebuild_padding function in fastdeploy/model_executor/pre_and_post_process.py is called frequently during model execution. The original implementation checked current_platform.is_*() and performed local imports on every call, adding unnecessary overhead.

Modifications

• Optimized rebuild_padding to resolve the platform-specific implementation once (lazy initialization) and cache it in a module-level global variable _rebuild_padding_impl.
• Introduced wrapper functions for platforms (DCU, GCU, CPU) that require fewer arguments than the standard interface, unifying the call signature.

Usage or Command

No changes to usage or commands. The optimization is internal.

Accuracy Tests

• Verified functionally using a mocked unit test (tests/test_rebuild_padding_mock.py - deleted after verification) which confirmed that the platform check is performed only once per process lifetime.
• The logic preserves all original branching and argument handling.

Checklist

• I have read the CONTRIBUTING document.
• I have checked the code style.
• I have run the tests and they pass.

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PR created automatically by Jules for task 15703239940177532441 started by @ZeyuChen

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