Revisit torch._six.string_classes removal (#94709) (#97863)

Revisit `torch._six.string_classes` (which is `(str, bytes)`) removal: `isinstance(obj, string_classes) -> isinstance(obj, str)`.

Both `str` and `bytes` are `Sequence` classes.

```python
In [1]: from typing import Sequence

In [2]: issubclass(bytes, Sequence)
Out[2]: True

In [3]: issubclass(str, Sequence)
Out[3]: True
```

Re-add `bytes` to type guards like:

```python
def is_seq(obj):
    return isinstance(obj, Sequence) and not isinstance(obj, (str, bytes))
```

Ref:

- https://github.com/pytorch/pytorch/pull/94709#issuecomment-1487282912
- #97737
- #97789
Pull Request resolved: https://github.com/pytorch/pytorch/pull/97863
Approved by: https://github.com/Skylion007, https://github.com/albanD

torch/distributed/rendezvous.py

@@ -90,7 +90,7 @@ def _rendezvous_helper(url: str, rank: int, world_size_opt: Optional[int], **kwa
 
 
 def rendezvous(url: str, rank: int = -1, world_size: int = -1, **kwargs):
-    if not isinstance(url, str):
+    if not isinstance(url, (str, bytes)):
         raise RuntimeError("`url` must be a string. {}: {}".format(type(url), url))
 
     if not isinstance(rank, numbers.Integral):