[run_clm] clarify why we get the tokenizer warning on long input
Solving https://github.com/huggingface/transformers/issues/11108 this PR adds a clarification of why the warning is printed by the tokenizer when `run_clm.py` sends a huge input to tokenize against a short `block_size`. It's not great, but at least now the user will know that the warning is not warranted to be a warning in this particular situation.

> [WARNING|tokenization_utils_base.py:3138] 2021-04-06 21:29:29,790 >> Token indices sequence length is longer than the specified maximum sequence length for this model (1462828 > 1024). Running this sequence through the model will result in indexing errors

So after this PR the we end up an extra warning:

```
[WARNING|tokenization_utils_base.py:3143] 2021-04-07 21:09:22,144 >> Token indices sequence length is longer than the specified maximum sequence length for this model (1462828 > 1024). Running this sequence through the model will result in indexing errors
[WARNING|run_clm.py:326] 2021-04-07 21:13:14,300 >> ^^^^^^^^^^^^^^^^ Please ignore the warning above - it's just a long input
```

The correct solution would be to redesign the API to notify the tokenizer that in some cases the input doesn't have to be less than `block_size`.

Fixes: https://github.com/huggingface/transformers/issues/11108

@sgugger 