[QDP] Fix streaming host-to-device copy size

[viiccwen]

Related Issues

Closes #1463

Changes

• Bug fix
• New feature
• Refactoring
• Documentation
• Test
• CI/CD pipeline
• Other

Why

The streaming Parquet encoder passed an f64 element count to a helper that expects a byte count. Consequently, amplitude and angle encoders copied only one eighth of every input chunk to GPU staging memory. The CUDA copy completed successfully, so callers received a state tensor with no error even though its amplitudes were incorrect.

Existing integration tests only checked the returned DLPack shape and therefore did not detect this numerical corruption.

How

• Convert the Parquet chunk length from f64 elements to bytes before calling PipelineContext::async_copy_to_device.
• Use checked_mul(size_of::<f64>()) and return a memory-allocation error if the byte-count calculation overflows.
• Extend the existing angle streaming integration test instead of adding a separate overlapping test:
  • encode three distinct angle samples from Parquet;
  • download the returned complex128 DLPack tensor;
  • compare all 24 complex amplitudes with the analytical angle-encoding
    formula;
  • retain the existing shape and cleanup checks.

The regression test was also run against the previous implementation. It failed as expected: state[0] was 1.0, while the analytical result was 0.9316157966884513. The test passes with this change.

Checklist

• Added or updated unit tests for all changes
• Added or updated documentation for all changes (the copy-unit contract is documented inline; there is no user-facing API change)

[Copilot]

Pull request overview

Fixes a correctness bug in the QDP streaming Parquet GPU encoder where an element count (f64 elements) was incorrectly passed to a helper expecting a byte count, causing truncated H2D copies and silently corrupted amplitude/angle results. Adds a stronger integration test that validates numerical output (not just shape) to prevent regressions.

Changes:

• Convert streaming Parquet chunk length from element-count to byte-count before PipelineContext::async_copy_to_device, with overflow checking.
• Extend the existing GPU angle streaming integration test to download the produced complex128 DLPack tensor and validate all amplitudes against the analytical formula.
• Add a test helper to download complex128 CUDA DLPack tensors to host for verification.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
qdp/qdp-core/src/encoding/mod.rs	Fixes H2D copy length by converting element counts to byte counts with overflow handling.
qdp/qdp-core/tests/gpu_angle_encoding.rs	Strengthens the Parquet streaming angle test to validate numerical correctness by downloading and checking amplitudes.
qdp/qdp-core/tests/common/mod.rs	Adds a CUDA DLPack complex128 download helper used by the new/extended integration test.

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[rich7420]

LGTM.