Add possibility to switch between APEX and AMP in Trainer
# What does this PR do?

When PyTorch >= 1.6 is installed, Trainer is always using native AMP right now. This PR adds the option to switch between AMP and APEX, which can be useful:
- because of the memory leak in AMP fixed (fixed in 1.7.1 but present in 1.6)
- to benchmark APEX vs. AMP

It also simplifies a little bit the internal of Trainer with those.
This PR also removes `_use_ddp_no_sync` since presumably `transformers` no longer supports pytorch < 1.2<|||||>There was one nit that you agreed with but didn't integrate - but I'm fine if it remains as merged - just a potential for divergence down the road...<|||||>Oh, which one did I miss?<|||||>https://github.com/huggingface/transformers/pull/9137#discussion_r543638157
<|||||>Argh, one sec - I see what happened - that wasn't what I meant - sorry for not being clear. `choices` is crucial here - since you don't validate the user-provided values - this is error-prone. 

I tried to suggest not repeating the options in the help comment - please let's have `choices` back, and duplicate them if you prefer the help to have the explicit repetition - thanks.<|||||>Fixed directly on master in [this commit](https://github.com/huggingface/transformers/commit/51adb97cd644a5840d971868d18c1d436fd6ff5d).<|||||>That's perfect. Thank you, @sgugger!