[MPS] Add naive std_mean implementation (#119777)
By just calling `std_mps` and `mean` in sequence
Move `var_mean` decomp to `ReduceOps.mm`, as it should be faster to skip dispatching to a Python, which one can validate by running the following script:
```python
from timeit import default_timer
import torch
from torch.utils.benchmark import Measurement, Timer
def bench_var_mean(
m, n, k,
dtype = torch.float32,
device:str = "cpu",
) -> Measurement:
setup = f"""
x = torch.rand({m}, {n}, {k}, dtype={dtype}, device="{device}")
"""
t = Timer(
stmt="torch.var_mean(x, dim=1)", setup=setup, language="python", timer=default_timer
)
return t.blocked_autorange()
for x in [100, 1000]:
rc = bench_var_mean(1000, x, 100, device="mps")
print(f"{x:5} : {rc.mean*1e6:.2f} usec")
```
which before the change reports 681 and 1268 usec and after 668 and 684 (which probably means that GPU is not saturated, but overhead from switching between native and interpretable runtimes are shorter.
Fixes https://github.com/pytorch/pytorch/issues/119663
TODOs:
- Refactor the codebase and implement proper composite function (that must be faster)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/119777
Approved by: https://github.com/albanD
