[FFI][Fix] Update datatype registry calls to the new paths

[cbalint13]

This updates the python calls to the new registry home.

See the new paths:

tvm/src/target/datatype/registry.cc

Lines 34 to 46 in 60f5568

	.def_packed("dtype.register_custom_type",
	[](ffi::PackedArgs args, ffi::Any* ret) {
	datatype::Registry::Global()->Register(
	args[0].cast<std::string>(), static_cast<uint8_t>(args[1].cast<int>()));
	})
	.def_packed("dtype.get_custom_type_code",
	[](ffi::PackedArgs args, ffi::Any* ret) {
	*ret = datatype::Registry::Global()->GetTypeCode(args[0].cast<std::string>());
	})
	.def_packed("dtype.get_custom_type_name",
	[](ffi::PackedArgs args, ffi::Any* ret) {
	*ret = Registry::Global()->GetTypeName(args[0].cast<int>());
	})

Usage errors:

  File "/usr/lib64/python3.14/site-packages/tvm/target/datatype.py", line 58, in register
    tvm.runtime._ffi_api._datatype_register(type_name, type_code)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: module 'tvm.runtime._ffi_api' has no attribute '_datatype_register'

Cc @tqchen