Add slow path to `getCustomClassTypeImpl` (#68717)
Summary:
This fixes custom class registration issue when `typeid` is not guaranteed to be unique across multiple libraries, which is the case for libc++ runtime on MacOS 11 in particular for M1
From [libcxx/include/typeinfo](https://github.com/llvm-mirror/libcxx/blob/78d6a7767ed57b50122a161b91f59f19c9bd0d19/include/typeinfo#L139):
```
// -------------------------------------------------------------------------- //
// NonUniqueARMRTTIBit
// -------------------------------------------------------------------------- //
// This implementation of type_info does not assume always a unique copy of
// the RTTI for a given type inside a program. It packs the pointer to the
// type name into a uintptr_t and reserves the high bit of that pointer (which
// is assumed to be free for use under the ABI in use) to represent whether
// that specific copy of the RTTI can be assumed unique inside the program.
// To implement equality-comparison of type_infos, we check whether BOTH
// type_infos are guaranteed unique, and if so, we simply compare the addresses
// of their type names instead of doing a deep string comparison, which is
// faster. If at least one of the type_infos can't guarantee uniqueness, we
// have no choice but to fall back to a deep string comparison.
```
But `std::type_index` hash is computed always assuming that implementation is unique
By adding a slow path this problem can be fixed in those scenarios.
Fixes https://github.com/pytorch/pytorch/issues/68039
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68717
Reviewed By: seemethere
Differential Revision: D32605187
Pulled By: malfet
fbshipit-source-id: 8d50e56885b8c97dad3bc34a69c47ef879456dd1
