Avoid aggregate graph access descriptor initialization

[rwgk]

Description

Follow-up to #2683.

PR #2683 centralizes CUmemLocation construction, but the graph allocation path still embeds the returned location in a CUmemAccessDesc aggregate constructor. With the CUDA 13.4 generated declaration, that nested aggregate causes Cython to emit an invalid conversion for the added localized union arm.

Declare the access descriptor separately and assign its location and flags fields before adding it to the descriptor vector. This keeps the helper introduced by #2683 and preserves the existing runtime behavior.

Please see ctk-next PR 539 for background on how this escaped attention while validating #2683.

Validation

• pre-commit run --all-files
• The equivalent change is validated against the CUDA 13.4 generated bindings in ctk-next PR 539.

Checklist

• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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[rwgk]

/ok to test

[rwgk]

Additional rationale for merging this PR

The need for this change appears to arise from a Cython code-generation issue exposed by cybind's representation of CUDA's anonymous nested struct:

• The aggregate initializer causes Cython to emit an unused struct-to-Python conversion helper.
• That helper attempts to convert the actual anonymous C++ type of localized to cybind's synthetic named struct type, which the generated C++ compiler correctly rejects as incompatible.
• Field-by-field initialization compiles because it avoids generating that conversion helper.
• C struct aggregate initialization is supported Cython syntax, so the original call site was not misusing Cython. See the Cython language basics documentation.

The field-by-field initialization in this PR is therefore a narrowly scoped workaround that preserves the intended descriptor contents and behavior.

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