Fix dangling pointer in TextTokenGenerator non-kv-cache path (#18725)

[kirklandsign]

Summary:

In the non-kv-cache branch of TextTokenGenerator::generate(), push_back()
on token_data can trigger vector reallocation, but the tensor created via
from_blob still points to the old data address. resize_tensor_ptr only
updates shape metadata, not the data pointer, resulting in a dangling
pointer.

Fix by pre-allocating the vector with reserve() before creating the
tensor, ensuring push_back never triggers reallocation during the
generate loop.

Reviewed By: larryliu0820

Differential Revision: D99408541

[meta-codesync]

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[pytorch-bot]

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❌ 2 New Failures, 9 Pending

As of commit ab0446e with merge base 19bbeac ():

NEW FAILURES - The following jobs have failed:

• Cadence Build & Test / cpu-test / test-ops / test-ops (gh)
  examples/cadence/operators/test_g3_ops.py::ATenOpTestCases::test_g3_neg_out_2
• pull / unittest-editable / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv3_model

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[Copilot]

Pull request overview

Fixes a memory-safety issue in the LLM token generation loop when KV-cache is disabled: from_blob() wraps token_data.data(), but subsequent push_back() could reallocate the vector and leave the tensor with a dangling data pointer.

Changes:

• Pre-reserve token_data capacity in the non-KV-cache path to prevent reallocation during generation.

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[larryliu0820]

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