make THPPointer have explicit constructors

[killeent]

Previously, THPPointer did not have its constructors marked as explicit. As a result, something like:

THTensor *someIntermediateResult = THTensor_(new)(LIBRARY_STATE_NOARGS);
// do some things to someIntermediateResult
THPTensor wrapped = someIntermediateResult; // calls implicit constructor

The fact that THPTensor takes "ownership" of the THTensor in this case is obfuscated, which could lead to double frees.

This diff marks all the THPTensor constructors explicit, preventing this type of implicit constructor from a pointer. I followed it with the following codemod:

codemod -m -d torch/ --extensions h,hpp,cpp,cwrap \
    '(TH[a-zA-Z_]+Ptr [a-zA-Z_]+) = ([^;]*);'  \
    '\1(\2);'

To replace all assignment calls with calls to the explicit constructor. There were a couple of other places where I need to fix this, e.g. in the THPPlugin.py allocation template.

Test Plan: Run Tests locally

[killeent]

@colesbury I think it was due to the fact that the move constructor had been marked explicit. By removing the keyword, I no longer need to pass the Tensor as a raw pointer return type in order for it to compile.