[Static Runtime] Enforce proper output dtype for many ops
Summary:
We previously had lots of ops with implementations like this:
```
if (p_node->Output(0).isNone()) {
p_node->Output(0) = create_empty_like(input_0);
}
...
auto& out = p_node->Output(0);
some_func_out(inputs, out);
```
This would make the output have the correct shape. But it would
also take the dtype of `input_0`, which is not always correct.
This change transforms these blocks to:
```
if (p_node->Output(0).isNone()) {
p_node->Output(0) = some_func(inputs)
} else {
...
auto& out = p_node->Output(0);
some_func_out(inputs, out);
}
```
This gives the output the correct shape and dtype.
Test Plan: `buck test //caffe2/benchmarks/static_runtime:static_runtime_cpptest`
Reviewed By: hlu1
Differential Revision: D29887367
fbshipit-source-id: cef04bfa52ec082ad3a9a32aa27c44e275c6b24c
