Improve CPUAllocator OOM message (#20618)
Summary:
Spotted while debugging some problem
Before
```
>>> torch.empty(10**15)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
RuntimeError: [enforce fail at CPUAllocator.cpp:56] posix_memalign(&data, gAlignment, nbytes) == 0. 12 vs 0
```
After
```
>>> torch.empty(10**15)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
RuntimeError: [enforce fail at CPUAllocator.cpp:65] . DefaultCPUAllocator: can't allocate memory: you tried to allocate 4000000000000000 bytes. Error code 12 (Cannot allocate memory)
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20618
Reviewed By: ezyang
Differential Revision: D15390400
Pulled By: dzhulgakov
fbshipit-source-id: 31f448303e4bd5f8c2bad8ca0f05bcece22a4b5e
