[Benchmark] Add encoder decoder to benchmark and clean labels
This PR cleans the benchmark utils a bit more:
- tracing is made independent from CPU memory benchmarking
- possibility to benchmark encoder-decoder models is added
- 3 new tests
- general refactoring
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4810?src=pr&el=h1) Report
> Merging [#4810](https://codecov.io/gh/huggingface/transformers/pull/4810?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/b6f365a8ed32eca20034084f74450723414b5de6&el=desc) will **increase** coverage by `1.19%`.
> The diff coverage is `76.19%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4810/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4810?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4810      +/-   ##
==========================================
+ Coverage   75.36%   76.55%   +1.19%     
==========================================
  Files         128      128              
  Lines       21497    21531      +34     
==========================================
+ Hits        16201    16484     +283     
+ Misses       5296     5047     -249     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4810?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/benchmark/benchmark.py](https://codecov.io/gh/huggingface/transformers/pull/4810/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9iZW5jaG1hcmsvYmVuY2htYXJrLnB5) | `68.68% <70.00%> (+26.22%)` | :arrow_up: |
| [src/transformers/benchmark/benchmark\_args\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4810/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9iZW5jaG1hcmsvYmVuY2htYXJrX2FyZ3NfdXRpbHMucHk=) | `96.87% <100.00%> (+0.10%)` | :arrow_up: |
| [src/transformers/benchmark/benchmark\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4810/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9iZW5jaG1hcmsvYmVuY2htYXJrX3V0aWxzLnB5) | `67.24% <100.00%> (+23.67%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4810/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.20% <0.00%> (ø)` | |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4810/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.15% <0.00%> (+0.24%)` | :arrow_up: |
| [src/transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/4810/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19ncHQyLnB5) | `85.75% <0.00%> (+54.43%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4810?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4810?src=pr&el=footer). Last update [b6f365a...49713d9](https://codecov.io/gh/huggingface/transformers/pull/4810?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
