Support `logging_ratio`, `save_ratio`, and `eval_ratio` (like for `warmup_ratio`)
### Feature request

I would love if `TrainingArguments` and the Huggingface `Trainer` would support `logging_ratio`, `save_ratio`, and `eval_ratio` arguments (complementing `logging_steps`, `save_steps`, and `eval_steps`). If the `*_ratio` argument is set to e.g. `0.1`, logging/saving/eval would be done every `0.1 * total_training_steps`. This is already done for `warmup_ratio` and `warmup_steps`.

### Motivation

When dealing with many different tasks and datasets, it can be frustrating to have to calculate different appropriate `logging_steps` etc. for each individual dataset. This proposal would enable a unified, simple and concise way to solve this problem.

### Your contribution

I realize this might not be trivial to fully integrate, but hopefully, we can take `warmup_steps` and `warmup_ratio` as a reference. Depending on how deep the required changes are, I can also submit a PR (with some pointers on what to look out for). 
We already have 96 training arguments though, and that would make three more of all users to learn :-/<|||||>Another option would be to allow `float` inputs for `logging_steps`, `save_steps`, `eval_steps` and interpret all inputs `<1.0` as a ratio (and throw an error if inputs `>1.0` are not integers). 

But yes there is a tradeoff with too much complexity<|||||>I would prefer that solution actually, even if the naming is not perfect.<|||||>Going over the code a bit, it seems like we would have to wait until after this if statement in `_inner_training_loop` to set the correct values based on the `max_steps` that gets calculated there + some additional guards in the `__post_init__` of the `TrainingArguments`. 
https://github.com/huggingface/transformers/blob/04ab5605fbb4ef207b10bf2772d88c53fc242e83/src/transformers/trainer.py#L1683-L1711
At first glance, it looks like the setting of when things get logged/saved/evaluated in `DefaultFlowCallback` should work out-of-the-box with this change.
I'm willing to contribute the changes once I find some time, does the general plan sound reasonable?<|||||>Yes it does. I'll be looking forward to your PR!