[FFI][REFACTOR] Direct structural APIs to tvm-ffi

[tqchen]

Summary

Python callers should reach the canonical tvm-ffi structural helpers directly instead of going through a TVM-side redirect layer. This makes the public tvm.ir bindings exact aliases of the tvm_ffi APIs and exposes get_first_structural_mismatch from tvm.ir.

Main changes:

• Import structural_equal, get_first_structural_mismatch, and structural_hash directly from tvm_ffi
• Remove the pure wrappers from tvm.ir.base while keeping assert_structural_equal's TVM-specific formatting
• Update mismatch tests and add identity coverage for the direct bindings

[gemini-code-assist]

Code Review

This pull request refactors the structural equality and hashing APIs in TVM's IR module. It removes the redundant Python wrapper functions structural_equal, get_first_structural_mismatch, and structural_hash from python/tvm/ir/base.py and instead exposes the direct FFI implementations from tvm_ffi via python/tvm/ir/__init__.py. Test files and internal references have been updated to use the new import paths, and a new test has been added to verify that these APIs correctly redirect to their FFI counterparts. There are no review comments, and I have no feedback to provide.

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