cuda.core: materialize NVRTC source to a real path when debug info is enabled

[rparolin]

Summary

When a user compiles a kernel via cuda.core.Program with debug=True or lineinfo=True, cuda-gdb cannot show source lines because the program name= defaults to the literal string "default_program" and no file with that name exists on disk. The debugger reports default_program:10 — No such file or directory and users lose source-level debugging.

This is tracked in NVBugs 6420444 (originally reported by Linaro). The customer was unblocked by a workaround: pass a real file path as name= and write the source there manually. We want this to work by default.

Proposed fix

When debug=True or lineinfo=True is set on Program(...):

1. Derive a meaningful filename from the caller's Python frame + a sanitized kernel identifier (e.g. train_py__matmul.cu).
2. Materialize the NVRTC source to a temp file at that path.
3. Pass that real path as NVRTC's name=.
4. Clean up the temp file on program destruction.

When neither debug flag is set, behavior is unchanged (no disk I/O, same label as today).

Impact on existing users

• No API or signature changes.
• Users who already pass name= explicitly: zero change (their value wins).
• Users who don't request debug info: zero change (no temp file written).
• Users who do request debug info: the label in cuda-gdb / traces / logs changes from default_program to a real path. Cosmetic; easily searchable.

Edge cases to handle

• Read-only /tmp or sandboxed environments: fall back to today's label-only behavior on write failure, do not crash.
• Concurrent compiles of the same-named kernel: use unique suffix (tempfile.mkstemp or PID+hash).
• Respect TMPDIR; user-only permissions on the temp file.
• Cleanup on both normal destruction and process exit.

Acceptance criteria

• cuda-gdb shows source lines when stepping into a kernel compiled with debug=True, without any user-side workaround.
• Existing behavior preserved when debug flags are off.
• Graceful fallback when the filesystem isn't writable.
• Tests covering: debug-on path, debug-off path, read-only-tmp fallback, concurrent compile.

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Related: NVBugs 6420444