Summary:
Fixes #31271

This fixes copy kernel speed regression introduced in #29631.

The previous implementation forces the compiler to instantiate `static_cast_with_inter_type` because it is passed as an argument of a function. This behavior makes it impossible for compilers to do optimizations like automatic vectorization, and, function call itself is expensive compared to a single casting instruction.

To check the change, run
```
readelf -Ws /home/xgao/anaconda3/lib/python3.7/site-packages/torch/lib/libtorch_cpu.so | grep static_cast_with_inter_type
```

On nightly build, we have output
```
168217: 0000000001852bf0     5 FUNC    LOCAL  DEFAULT    9 _ZN3c1027static_cast_with_inter_typeIsdE5applyEd
168816: 0000000001852d30    33 FUNC    LOCAL  DEFAULT    9 _ZN3c1027static_cast_with_inter_typeISt7complexIfEaE5applyEa
168843: 00000000018531f0     7 FUNC    LOCAL  DEFAULT    9 _ZN3c1027static_cast_with_inter_typeIblE5applyEl
168930: 0000000001852c20     3 FUNC    LOCAL  DEFAULT    9 _ZN3c1027static_cast_with_inter_typeIslE5applyEl
168935: 00000000018528d0   124 FUNC    LOCAL  DEFAULT    9 _ZN3c1027static_cast_with_inter_typeIfNS_4HalfEE5applyES1_
169023: 0000000001852f30    17 FUNC    LOCAL  DEFAULT    9 _ZN3c1027static_cast_with_inter_typeISt7complexIdEhE5applyEh
169713: 00000000018525c0     3 FUNC    LOCAL  DEFAULT    9 _ZN3c1027static_cast_with_inter_typeIahE5applyEh
170033: 0000000001852c10     3 FUNC    LOCAL  DEFAULT    9 _ZN3c1027static_cast_with_inter_typeIsiE5applyEi
170105: 0000000001852bd0     5 FUNC    LOCAL  DEFAULT    9 _ZN3c1027static_cast_with_inter_typeIshE5applyEh
170980: 0000000001852fc0    27 FUNC    LOCAL  DEFAULT    9 _ZN3c1027static_cast_with_inter_typeISt7complexIdES1_IfEE5applyES3_
171398: 0000000001852810    13 FUNC    LOCAL  DEFAULT    9 _ZN3c1027static_cast_with_inter_typeIdbE5applyEb
171574: 00000000018532e0    35 FUNC    LOCAL  DEFAULT    9 _ZN3c1027static_cast_with_inter_typeIbNS_8BFloat16EE5applyES1_
171734: 0000000001852b20     6 FUNC    LOCAL  DEFAULT    9 _ZN3c1027static_cast_with_inter_typeIlSt7complexIdEE5applyES2_
172422: 0000000001853350    54 FUNC    LOCAL  DEFAULT    9 _ZN3c1027static_cast_with_inter_typeINS_8BFloat16EaE5applyEa
172704: 00000000018533c0    38 FUNC    LOCAL  DEFAULT    9 _ZN3c1027static_cast_with_inter_typeINS_8BFloat16EfE5applyEf
172976: 0000000001852890    10 FUNC    LOCAL  DEFAULT    9 _ZN3c1027static_cast_with_inter_typeIflE5applyEl
173038: 0000000001852f80     9 FUNC    LOCAL  DEFAULT    9 _ZN3c1027static_cast_with_inter_typeISt7complexIdEfE5applyEf
173329: 00000000018531c0    20 FUNC    LOCAL  DEFAULT    9 _ZN3c1027static_cast_with_inter_typeIbfE5applyEf
173779: 00000000018524d0     3 FUNC    LOCAL  DEFAULT    9 _ZN3c1027static_cast_with_inter_typeIhiE5applyEi
174032: 0000000001852960    14 FUNC    LOCAL  DEFAULT    9 _ZN3c1027static_cast_with_inter_typeIfNS_8BFloat16EE5applyES1_
174334: 0000000001852d60    29 FUNC    LOCAL  DEFAULT    9 _ZN3c1027static_cast_with_inter_typeISt7complexIfEdE5applyEd
174470: 0000000001852c60   124 FUNC    LOCAL  DEFAULT    9 _ZN3c1027static_cast_with_inter_typeIsNS_4HalfEE5applyES1_
174770: 0000000001852bc0    15 FUNC    LOCAL  DEFAULT    9 _ZN3c1027static_cast_with_inter_typeIlNS_8BFloat16EE5applyES1_
176408: 0000000001853980   144 FUNC    LOCAL  DEFAULT    9 _ZN3c1027static_cast_with_inter_typeINS_4HalfEbE5applyEb
176475: 0000000001852790   128 FUNC    LOCAL  DEFAULT    9 _ZN3c1027static_cast_with_inter_typeIdNS_4HalfEE5applyES1_
....
```

And after this PR, we get empty output
```
```
Pull Request resolved: #31279

Differential Revision: D19075587

Pulled By: ngimel

fbshipit-source-id: c20088241f39fa40c1d055f0a46eb5b9ece52e71