[Bugfix] Fix cuda event usage with CPU model runner

[bigPYJ1151]

Purpose

• Add _EventPlaceholder to workaround CUDA event usage with CPUModelRunner
• Replace GPU tensor in CpuGpuBuffer.

Test Plan

CI tests

Test Result

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request introduces a fix for using CUDA events with the CPU model runner by introducing a _torch_cuda_wrapper to monkey-patch torch.cuda.Event. The changes look good overall, but there is a critical issue in the implementation of _torch_cuda_wrapper that could lead to a crash in environments without CUDA support. I've provided a suggestion to make it more robust. The other changes, including using self.pin_memory and refactoring tensor handling with CpuGpuBuffer, are solid improvements.