[MISC] Add miss Type2Str and remove compile warnings

[comaniac]

After #10326, we now require the argument type of PackFunc to have a corresponding template of Type2Str. Otherwise we will get the following error during compilation (note that my argument type is DLDataType).

include/tvm/runtime/packed_func.h:1400:22: note: candidate: template<class> static std::__cxx11::string tvm::runtime::detail::type2str::Type2Str<T>::v() [with <template-parameter-2-1> = <template-parameter-1-1>; T = DLDataType]
   static std::string v() {
                      ^
/include/tvm/runtime/packed_func.h:1400:22: note:   template argument deduction/substitution failed

This PR adds a Type2Str template for DLDataType to remove my blocker. However, this doesn't solve the root problem. Ideally, we should not error out only if the Type2Str template is missing but the argument type of a PackFunc is actually supported. A straightforward solution to this issue is adding all templates for the types supported by TVMArgValue (e.g., intX_t, uintX_t for X in 8, 16, 32, 64; basically everything covered in https://github.com/apache/tvm/blob/main/include/tvm/runtime/packed_func.h#L1225). While it is tedious, I'm wondering if we could have a default template that simply prints "unknown_type" for non-ObjectRef types. It might be possible to leverage std::enable_if_t<!std::is_base_of<ObjectRef, T>::value>, but I'm not sure how to achieve it in the case of Type2Str struct.

(nit) In addition, this PR also removes some compiler warnings reported by Clang:

1. size_t is always larger or equal than zero, so no need to check idx >= 0.
2. The derived mutator and visitor don't implement all virtual functions.

cc @cyx-6 @junrushao1994

[junrushao]

LGTM!