Fix inconsistent results of string `split` func on JIT mode (#38772)
Summary:
Resolve https://github.com/pytorch/pytorch/issues/38207
Below is the description of split function according to [Python doc](https://docs.python.org/3.8/library/stdtypes.html?highlight=split#str.split).
```
If sep is not specified or is None, a different splitting algorithm is applied:
runs of consecutive whitespace are regarded as a single separator,
and the result will contain no empty strings at the start or end
if the string has leading or trailing whitespace.
```
The logic to handle both none and empty separators is added in register_string_ops.cpp as fix.
Signed-off-by: Xiong Wei <xiongw.fnst@cn.fujitsu.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38772
Differential Revision: D21789612
Pulled By: suo
fbshipit-source-id: 4dfd74eda71e0bfd757378daedc927a4a63ec0e4
