[CUDA] Preserve fallback when remerging PTEs

[Gasoonjia]

This PR fixes the issue when we try to merge multi-sm PTEs with regular ones. Also add relevant tests.

[pytorch-bot]

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❌ 3 New Failures, 20 Pending, 1 Unclassified Failure

As of commit 0105659 with merge base ad2de69 ():

NEW FAILURES - The following jobs have failed:

• pull / test-lora-linux / linux-job (gh)
  RuntimeError: Command bash /__w/_temp/exec_script failed with exit code 1
• pull / test-lora-multimethod-linux / linux-job (gh)
  RuntimeError: Command bash /__w/_temp/exec_script failed with exit code 1
• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command bash /__w/_temp/exec_script failed with exit code 1

UNCLASSIFIED FAILURE - DrCI could not classify the following job because the workflow did not run on the merge base. The failure may be pre-existing on trunk or introduced by this PR:

• Test CUDA Builds / test-model-cuda-e2e-Qwen3.5-35B-A3B-HQQ-INT4-quantized-int4-tile-packed / linux-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: Command bash /__w/_temp/exec_script failed with exit code 1

This comment was automatically generated by Dr. CI and updates every 15 minutes.

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