[test schedulers] adjust to test the first step's reading
As I was working on a new scheduler, it was difficult to match numbers since the first step's reading was dropped in `unwrap_schedule` wrappers (they were taking the measurement after stepping). This PR adjusts the wrappers to first take a reading and then step.

This PR also makes a small refactoring to move all the unwrapping into the script, so the test just compares 2 lists. (avoiding multiple `[l[0] for l in lrs_1]`)

The updated table is:
```
        scheds = {
            get_constant_schedule: ({}, [10.0] * self.num_steps),
            get_constant_schedule_with_warmup: (
                {"num_warmup_steps": 4},
                [0.0, 2.5, 5.0, 7.5, 10.0, 10.0, 10.0, 10.0, 10.0, 10.0],
            ),
            get_linear_schedule_with_warmup: (
                {**common_kwargs},
                [0.0, 5.0, 10.0, 8.75, 7.5, 6.25, 5.0, 3.75, 2.5, 1.25],
            ),
            get_cosine_schedule_with_warmup: (
                {**common_kwargs},
                [0.0, 5.0, 10.0, 9.61, 8.53, 6.91, 5.0, 3.08, 1.46, 0.38],
            ),
            get_cosine_with_hard_restarts_schedule_with_warmup: (
                {**common_kwargs, "num_cycles": 2},
                [0.0, 5.0, 10.0, 8.53, 5.0, 1.46, 10.0, 8.53, 5.0, 1.46],
            ),
            get_polynomial_decay_schedule_with_warmup: (
                {**common_kwargs, "power": 2.0, "lr_end": 1e-7},
                [0.0, 5.0, 10.0, 7.656, 5.625, 3.906, 2.5, 1.406, 0.625, 0.156],
            ),
        }
```
Unrelated to the changes suggestion in this PR, it exposes 2 minor issues:

1. We definitely have a one off problem there, as the last step's reading is one reading too early (which this change exposes) - it doesn't complete the intended cycle. This is probably unimportant for 100s of steps, but it definitely stands out when developing a new scheduler.

To illustrate, see this change in reported number for `get_polynomial_decay_schedule_with_warmup`:

```
-                [5.0, 10.0, 7.656, 5.625, 3.906, 2.5, 1.406, 0.625, 0.156, 1e-07],
+                [0.0, 5.0, 10.0, 7.656, 5.625, 3.906, 2.5, 1.406, 0.625, 0.156],
```
the expected last step of `1e-07` is not there. It never was.

2. Also the first step's reading is `0.0` in all schedulers, except in `get_constant_schedule`, so the first step does nothing. This can be fixed with a potentially added `min_lr=1e-7` to all schedulers, as it was suggested by @sshleifer in one of the recent scheduler-related PRs.

Let me know if this better fits into its own issue, as these issues have nothing to do with the PR itself. Or perhaps the 2 issues are just unimportant...
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6429?src=pr&el=h1) Report
> Merging [#6429](https://codecov.io/gh/huggingface/transformers/pull/6429?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/4ffea5ce2f4d154a3696b8fe2fb116fa09235700&el=desc) will **increase** coverage by `0.05%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6429/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6429?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6429      +/-   ##
==========================================
+ Coverage   79.89%   79.94%   +0.05%     
==========================================
  Files         153      153              
  Lines       27902    27902              
==========================================
+ Hits        22291    22307      +16     
+ Misses       5611     5595      -16     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6429?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/6429/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYWxiZXJ0LnB5) | `28.84% <0.00%> (-58.66%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/6429/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `64.47% <0.00%> (-32.95%)` | :arrow_down: |
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6429/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `90.40% <0.00%> (+0.40%)` | :arrow_up: |
| [src/transformers/tokenization\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/6429/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmVydC5weQ==) | `91.51% <0.00%> (+0.44%)` | :arrow_up: |
| [src/transformers/configuration\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6429/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3V0aWxzLnB5) | `96.59% <0.00%> (+0.68%)` | :arrow_up: |
| [src/transformers/tokenization\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/6429/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fb3BlbmFpLnB5) | `84.09% <0.00%> (+1.51%)` | :arrow_up: |
| [src/transformers/tokenization\_utils\_fast.py](https://codecov.io/gh/huggingface/transformers/pull/6429/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfZmFzdC5weQ==) | `94.28% <0.00%> (+2.14%)` | :arrow_up: |
| [src/transformers/tokenization\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/6429/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYXV0by5weQ==) | `97.72% <0.00%> (+2.27%)` | :arrow_up: |
| [src/transformers/tokenization\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/6429/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdHJhbnNmb194bC5weQ==) | `42.48% <0.00%> (+3.75%)` | :arrow_up: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6429/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.71% <0.00%> (+5.01%)` | :arrow_up: |
| ... and [3 more](https://codecov.io/gh/huggingface/transformers/pull/6429/diff?src=pr&el=tree-more) | |

------

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