clean up some runtime potential bugs

[billmguo]

Summary:

1. Zero cache on allocation (line 65-66): std::fill on cache_data_ and update_data_ after allocator_.allocate() —eliminates uninitialized memory garbage that varies across devices.
2. Zero cache on reset (line 191): std::fill on cache_data_ in reset() — ensures stale KV cache from a previous prompt is fully cleared, not just the position counters.
3. Zero padding in last prefill chunk (line 618-621): When batch_len < input_len, fill the tail of the input buffer with zeros — prevents stale tokens from a previous chunk leaking through the embedding layer. sa_runner.cpp
4. Call runner.reset() before each prompt in the multi-prompt loop, stdin prompt loop, and stdin tokens loop —ensures the KV cache, masks, and input_pos_ are fully reset between prompts

Differential Revision: D104615993

[pytorch-bot]

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❌ 2 New Failures, 1 Pending, 1 Unrelated Failure

As of commit 9e0d34d with merge base a49171d ():

NEW FAILURES - The following jobs have failed:

• Docathon Labels Sync / check-labels (gh)
  Process completed with exit code 1.
• pull / unittest-arm-backend-with-no-deps (test_pytest_models_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t 78a575e143acb47fe046c53b2af9e211022b75269411c90324ad933e954d9b11 /exec failed with exit code 1

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-moshi-linux / linux-job (gh) (matched linux rule in flaky-rules.json)
  E: Failed to fetch http://security.ubuntu.com/ubuntu/pool/main/o/openjpeg2/libopenjp2-7_2.4.0-6ubuntu0.4_amd64.deb 404 Not Found [IP: 172.66.152.176 80]

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[meta-codesync]

@billmguo has exported this pull request. If you are a Meta employee, you can view the originating Diff in D104615993.

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