[REFACTOR][TIR] Remove buffer type and axis separators

[tqchen]

Rationale

axes_separator and buffer_type (auto-broadcast buffers) add representation and propagation complexity that is not needed for flat-buffer use cases. This change removes those surfaces and standardizes buffer handling on the default flat layout. Backward compatibility for compact JSON containing the removed fields is intentionally out of scope.

Changes

• Remove the reflected buffer fields and their C++, Python, packed-function, TVMScript, and printer surfaces.
• Remove auto-broadcast binding and axis-separator propagation from TIR, S-TIR, Relax, and backend lowering.
• Simplify buffer flattening and reconstruction for the supported flat-buffer contract.
• Remove tests that only exercised the phased-out features while preserving default flat-buffer coverage.

[gemini-code-assist]

Code Review

This pull request removes the concepts of axis_separators and buffer_type (including kAutoBroadcast) from the TIR and Relax buffer representations, layout transformation passes, and scheduling APIs. This simplifies buffer definitions, constructors, and FFI bindings across C++ and Python, and cleans up associated tests and scheduling primitives such as SetAxisSeparator. I have no feedback to provide as there are no review comments.

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