

TITLE
Trainer incorrectly checks pytorch version

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## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version: 3.1.0
- Platform: Linux-3.10.0-862.el7.x86_64-x86_64-with-glibc2.27
- Python version: 3.8.5
- PyTorch version (GPU?): 1.6.0 (True)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: yes(not applicable)
- Using distributed or parallel set-up in script?: not applicable

### Who can help
@sgugger and @prajjwal1 (he added it, according to git blame)

## Information

I'm running token classification example on my own data, and i've faced trouble with fp16 training with torch 1.6.0.
Script says that i need apex installed to use fp16 option. However, apex is not required since torch 1.6.0 came out with native amp support. I've dived into trainer code, and found that there is version checking line: https://github.com/huggingface/transformers/blob/99cb924bfb6c4092bed9232bea3c242e27c6911f/src/transformers/trainer.py#L65

Apparently, it is slightly incorrect. There should be <= instead of <. So it will not try to import apex if torch version is greater OR EQUAL to 1.6

```python
import torch
from packaging import version

print(version.parse(torch.__version__) < version.parse("1.6")) # -> False
print(version.parse(torch.__version__) <= version.parse("1.6")) # -> True
```

## To reproduce
1. Install torch 1.6.0(and do not install apex)
2. clone repository
3. cd into examples/token-classification
4. add '--fp16' to the bottom of run.sh script
5. execute run.sh script

## Expected behavior
 Script works well on torch 1.6.0 without apex installed
