Fix bad import with PyTorch <= 1.4.1
# What does this PR do?

`trainer_pt_utils` imports `SAVE_STATE_WARNING` from PyTorch, which only exists in 1.5.0 or later. This fixes that problem.

Fixes #8232
FYI `SAVE_STATE_WARNING` has been removed 3 days ago: pytorch/pytorch#46813

So `transformers` needs to be recoded not to use that constant.

Looking at its use, this probably would suffice:

```
--- a/src/transformers/trainer_pt_utils.py
+++ b/src/transformers/trainer_pt_utils.py
@@ -34,7 +34,7 @@ from .utils import logging
 if is_torch_tpu_available():
     import torch_xla.core.xla_model as xm

-if version.parse(torch.__version__) <= version.parse("1.4.1"):
+if version.parse(torch.__version__) <= version.parse("1.4.1") or version.parse(torch.__version__) > version.parse("1.7.0"):
     SAVE_STATE_WARNING = ""
 else:
     from torch.optim.lr_scheduler import SAVE_STATE_WARNING
```

and perhaps adding a note why this was needed in first place.