fix pt-1.9.0 `add_` deprecation
This PR fixes a new pt-1.9.0 `add_` deprecation in several places. 

The deprecation warnings:
```
UserWarning: This overload of add_ is deprecated:
        add_(Number alpha, Tensor other)
Consider using one of the following signatures instead:
        add_(Tensor other, *, Number alpha) (Triggered internally at  /pytorch/torch/csrc/utils/python_arg_parser.cpp:1025.)
  exp_avg_sq.mul_(beta2t).add_(1.0 - beta2t, update)
```

The new API is at https://pytorch.org/docs/stable/generated/torch.Tensor.add_.html

## Backward compatibility alert

I tracked this API down to need pt-1.5.0 or higher

Requesting an easier way to do this kind of process: https://github.com/pytorch/pytorch/issues/60149

I still have no idea which minimal pytorch version `transformers` is meant to support. Merging this PR will push it at least to `torch>=1.5.0`. Last I [checked](https://github.com/huggingface/transformers/pull/7985) some 8 months ago we barely supported `torch>=1.4.0`.

If you're OK with `torch>=1.5.0` then we should revive and update [this](https://github.com/huggingface/transformers/pull/7985) or make a new PR or fix it here.

## Readability

Unfortunately since we have to use the named arg now, the autoformatter makes the code less readable, by forcing whitespace in the expression.

I wrote these as:
```
exp_avg.mul_(group["beta1"]).add_(update, alpha=1-group["beta1"])
```
to make it clear, it's an expression, but it made it into:
```
exp_avg.mul_(group["beta1"]).add_(update, alpha=1 - group["beta1"])
```
now it looks like alpha is 1. grrrr. perhaps `()` are needed for improved readability. i.e.:
```
exp_avg.mul_(group["beta1"]).add_(update, alpha=(1 - group["beta1"]))
```
@sgugger, @LysandreJik 
Can we had an import error in AdaFactor to error if the version is les than 1.5 then? It seems the code is only there.<|||||>Sure, I'm just not sure where we are at `transformers`-wise with minimal pt version, so it might be simpler to require pt-1.5+, but the suggestion you made works too for now.

Would it help to add `()` for `alpha` as described in the last section of OP? <|||||>Yes, I missed that part. Adding parenthesis is fine!<|||||>@sgugger, it's in AdamW too - it's just whoever coded it hasn't checked back-compat (they didn't know), i.e. search for `add_` - so I think we either need a wrapper or cut off at pt-1.5.0 project-wise.

Found this now, as I was adding `()` for clarity. See the new diff.<|||||>as discussed on slack for now adding:
```
require_version("torch>=1.5.0")  # add_ with alpha
```
for AdamW and Adafactor.
