[REFACTOR][DataType] Phase out target custom datatype support

[tqchen]

Summary

The in-tree target custom datatype path adds maintenance surface while current development focuses on core datatypes. This PR phases out the built-in registry/lowering implementation and keeps the core dtype behavior intact.

• Remove the target/datatype implementation, BYODT posit build option, and related Python helpers
• Remove the custom datatype lowering pass from TIRX and S-TIR finalization pipelines
• Simplify remaining TIRX dtype handling back to built-in/core datatypes

[gemini-code-assist]

Code Review

This pull request completely removes the "Bring Your Own Datatypes" (BYODT) custom datatype framework from TVM, including its CMake configurations, Docker dependencies, Python bindings, C++ registry, custom datatype lowering passes, and associated third-party wrappers. Feedback on the changes suggests that the check for DataType::kCustomBegin in src/target/llvm/codegen_llvm.cc was missed and should be removed as it is now obsolete.

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[gemini-code-assist]

While updating this comment to reflect the removal of custom datatypes, the check on line 603 (primtype->dtype.code() >= DataType::kCustomBegin) should also be removed as it is now obsolete.