Change to use relative imports in some files & Add python prompt symbols to example codes
Author: soheeyang
Pull request: yes

- Moved transformers package import statements to relative imports in some files as in https://github.com/huggingface/transformers/pull/5796
- Added python prompt symbols in front of the example codes as in most of the codebase

This is a resubmitted version of https://github.com/huggingface/transformers/pull/7188; I closed the previous PR because I used the wrong versions of style libraries.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7202?src=pr&el=h1) Report
> Merging [#7202](https://codecov.io/gh/huggingface/transformers/pull/7202?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/0203ad43bcd0b29423dec6ca1a58ed58300f0d61?el=desc) will **decrease** coverage by `2.45%`.
> The diff coverage is `71.42%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/7202/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/7202?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #7202      +/-   ##
==========================================
- Coverage   80.86%   78.41%   -2.46%     
==========================================
  Files         169      169              
  Lines       32293    32293              
==========================================
- Hits        26114    25322     -792     
- Misses       6179     6971     +792     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/7202?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/7202/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hdXRvLnB5) | `82.29% <ø> (ø)` | |
| [src/transformers/modeling\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/7202/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iYXJ0LnB5) | `94.44% <ø> (+0.67%)` | :arrow_up: |
| [src/transformers/modeling\_dpr.py](https://codecov.io/gh/huggingface/transformers/pull/7202/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19kcHIucHk=) | `97.83% <ø> (ø)` | |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/7202/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `72.25% <ø> (-10.00%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/7202/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9hbGJlcnQucHk=) | `90.92% <ø> (ø)` | |
| [src/transformers/modeling\_tf\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/7202/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9hdXRvLnB5) | `72.36% <ø> (ø)` | |
| [src/transformers/modeling\_tf\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/7202/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9iZXJ0LnB5) | `98.90% <ø> (ø)` | |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7202/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.33% <ø> (ø)` | |
| [src/transformers/modeling\_tf\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/7202/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl94bG5ldC5weQ==) | `92.25% <ø> (ø)` | |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7202/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `87.23% <0.00%> (ø)` | |
| ... and [26 more](https://codecov.io/gh/huggingface/transformers/pull/7202/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7202?src=pr&el=footer). Last update [0203ad4...da63e64](https://codecov.io/gh/huggingface/transformers/pull/7202?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).