AT_DISPATCH: Expose switch-case like macro syntax (#79978)
This expands the `AT_DISPATCH` macros to enable writing your own
`AT_DISPATCH_SWITCH` statements with multiple `AT_DISPATCH_CASE`
labels. So, where previously you may have written:
```cpp
if (iter.common_dtype() == kBool) {
my_bool_kernel(iter);
} else {
AT_DISPATCH_INTEGRAL_TYPES(iter.common_dtype(), "my_kernel", [&] {
...
});
}
```
You can now instead write
```cpp
AT_DISPATCH_SWITCH(iter.common_dtype(), "my_kernel",
AT_DISPATCH_CASE(kBool, [&] { my_kernel_bool(iter); })
AT_DISPATCH_CASE_INTEGRAL_TYPES([&] { ... })
);
```
The macro syntax is a bit ugly, however the benefits are:
- Greater flexibility, as the kernel code doesn't have to be shared
for all dtypes.
- Selective build and RECORD_KERNEL_FUNCTION work even for single
dtype specializations such as the bool case in the example.
- The compiler sees a single switch for all types, which should be
easier to optimize into a jump table.
- We also now get errors if the same scalar type is handled twice.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/79978
Approved by: https://github.com/ezyang
