Minor fix of warnings raised by -Wreturn-std-move and -Wself-assign-overloaded

[junrushao]

My clang keeps complaining about it.

Update:

For example,

/path/to/tvm/src/relay/pass/type_infer.cc:727:14: warning: local variable 'func' will be copied despite being returned by name [-Wreturn-std-move]
      return func;
             ^~~~
/path/to/tvm/src/relay/pass/type_infer.cc:727:14: note: call 'std::move' explicitly to avoid copying
      return func;
             ^~~~
             std::move(func)

The reason for this is that copy elision is not always turned on, so in this case, std::move is not a pessimization.

See this thraed in LLVM for details.

[tqchen]

hmm, I am not sure, in theory these can be optimized by copy elision and we should not explicit move

[junrushao]

I feel weird as well, but clang-7 keeps asking me to explicitly write std::move.

[junrushao]

Looks like “-Wreturn-std-move” reports stuff that are not eligible for RVO

[tqchen]

OK, I will merge them in for now, Thanks @junrushao1994