Pass Scalar by reference (#53583)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/53583

`Scalar` takes 32 bytes due to `c10::complex<double>`
requires aligning to 16 bytes. Passing Scalar by reference
shows about 1% improvements on instruction count.

All the changes in this commit are codemoded except for
the following 4 files (which code-gen signatures):
```
tools/codegen/api/cpp.py
tools/codegen/api/native.py
tools/codegen/api/structured.py
caffe2/contrib/aten/gen_op.py
```

# Codemode

## Main Step

For the codemod part, here is the main command used:
```
fastmod --extensions h '([a-zA-Z_+]\([^)]*,?\s*)Scalar (\w+)' '${1}const Scalar& ${2}'
fastmod --extensions h '([a-zA-Z_+]\([^)]*,?\s*)optional<Scalar> (\w+)' '${1}const optional<Scalar>& ${2}'
fastmod --extensions cpp '([a-zA-Z_+]\([^)]*,?\s*)Scalar (\w+)' '${1}const Scalar& ${2}'
fastmod --extensions cpp '([a-zA-Z_+]\([^)]*,?\s*)optional<Scalar> (\w+)' '${1}const optional<Scalar>& ${2}'
```

As you can tell, it codemods both `Scalar` and `optional<Scalar>`.  Apply these commands iteratively until reaching a fix-point (since one method signature might contain multiple `Scalar` parameter).

In retrospect, excluding `thrid_party` and `torch/csrc/jit` would be a good idea. (I revert it manually later, see https://github.com/pytorch/pytorch/pull/53479 as an reference).

## Pre-Step

Prior to applying the main command,  as some `Scalar` are presented as `at::Scalar` or `c10::Scalar`, so I codemod some of them in advance. Here is an incomplete list:
```
fastmod --extensions h '([a-zA-Z_+]\([^)]*,?\s*)at::Scalar (\w+)' '${1}const at::Scalar& ${2}'
fastmod --extensions cpp '([a-zA-Z_+]\([^)]*,?\s*)at::Scalar (\w+)' '${1}const at::Scalar& ${2}'
fastmod --extensions h '([a-zA-Z_+]\([^)]*,?\s*)c10::optional<Scalar> (\w+)' '${1}const c10::optional<Scalar>& ${2}'
fastmod --extensions cpp '([a-zA-Z_+]\([^)]*,?\s*)c10::optional<Scalar> (\w+)' '${1}const c10::optional<Scalar>& ${2}'
```

## Fixup
There are a couple of post codemod fixup. For example, `const Scalar` will be codemoded into `const const Scalar&`. `at:Scalar` will be codemoded into `at::const Scalar&`  (if `Pre-step` is not done comprehensively). Here is an incomplete list:
```
fastmod --extensions cpp 'const const Scalar' 'const Scalar'
fastmod --extensions h 'const const c10::optional<Scalar>' 'const c10::optional<Scalar>'
fastmod --extensions cpp 'const const c10::optional<Scalar>' 'const c10::optional<Scalar>'
fastmod 'at::const Scalar&' 'const at::Scalar&'
```

## Supplementary

`cu` and `mm` files also need to be codemoded, for example:

```
fastmod --extensions cu 'at::const Scalar&' 'const at::Scalar&'
fastmod --extensions mm '([a-zA-Z_+]\([^)]*,?\s*)Scalar (\w+)' '${1}const Scalar& ${2}'
```

Function pointers are not codemoded. Here is an incomplete list:

```
# Cover case: using index_fill_fn = void(*)(TensorIterator & iter, int64_t dim, int64_t self_dim_size, int64_t self_dim_stride, Scalar source);
fastmod --extensions h '(void\s*\(\s*\*\s*\)\([^)]*,?\s*)Scalar (\w+)' '${1}const Scalar& ${2}'

# Cover case: using softplus_fn = void (*)(TensorIterator&, Scalar, Scalar);
fastmod --extensions h '(void\s*\(\s*\*\s*\)\([^)]*,?\s*)Scalar([, \)])' '${1}const Scalar&${2}'
fastmod --extensions cpp '(void\s*\(\s*\*\s*\)\([^)]*,?\s*)Scalar([, \)])' '${1}const Scalar&${2}'
fastmod --extensions h '(void\s*\(\s*\*\s*\)\([^)]*,?\s*)optional<Scalar>([, \)])' '${1}const optional<Scalar>&${2}'
```

Some corner cases needs to be manually fixed.

ghstack-source-id: 123970306

Test Plan: Imported from OSS

Reviewed By: smessmer

Differential Revision: D26904445

fbshipit-source-id: 8d8a002af4b5125f153a32f03c6956be7ae5671d

tools/codegen/api/native.py

@@ -38,6 +38,10 @@ def argumenttype_type(t: Type, *, mutable: bool, binds: ArgName) -> CType:
             return ConstRefCType(tensor_type)
     elif str(t) == 'Tensor?[]':
         return ConstRefCType(BaseCType("c10::List<c10::optional<Tensor>>", binds))
+    elif str(t) == 'Scalar':
+        return ConstRefCType(BaseCType('Scalar', binds))
+    elif str(t) == 'Scalar?':
+        return ConstRefCType(OptionalCType(BaseCType('Scalar', binds)))
     return cpp.argumenttype_type(t, mutable=mutable, binds=binds)
 
 def returns_type(rs: Sequence[Return]) -> str: