add doctests for data2VecText
### Feature request

Enable doctests for data2VecText model, as part of https://github.com/huggingface/transformers/issues/16292

### Motivation

please see https://github.com/huggingface/transformers/issues/16292

### Your contribution

implement this feature
Hey @Ruihua-Fang,

Would you like to give it a try? :-)<|||||>Hey @patrickvonplaten , yep, Thanks :)
<|||||>Following the instruction in https://github.com/huggingface/transformers/issues/16292 as listed below:
Make sure to run the doc example doc test locally as described in https://github.com/huggingface/transformers/tree/master/docs#for-python-files
5 failed, 2 passed
see attached file for detailed 
[doctest_data2vec_text_errormsg.txt](https://github.com/huggingface/transformers/files/8730807/doctest_data2vec_text_errormsg.txt)
error messages 

p.s, for sanity check, I also run the doctest sample for the following:

bigbird_pegasus: all 5 tests passed
data2vec_audio in the same folder: 1 failed, 4 passed

error message for data2vec_audio:
[doctest] transformers.models.data2vec.modeling_data2vec_audio.Data2VecAudioForAudioFrameClassification.forward _______________________________________
1420 heads.
1421
1422 Example:
1423
1424 ```python
1425 >>> from transformers import Wav2Vec2FeatureExtractor, Data2VecAudioForAudioFrameClassification
1426 >>> from datasets import load_dataset
1427 >>> import torch
1428
1429 >>> dataset = load_dataset("hf-internal-testing/librispeech_asr_demo", "clean", split="validation")
Expected nothing
Got:
Downloading and preparing dataset librispeech_asr/clean to /home/ruihua/.cache/huggingface/datasets/hf-internal-testing___librispeech_asr/clean/2.1.0/d3bc4c2bc2078fcde3ad0f0f635862e4c0fef78ba94c4a34c4c250a097af240b...
Dataset librispeech_asr downloaded and prepared to /home/ruihua/.cache/huggingface/datasets/hf-internal-testing___librispeech_asr/clean/2.1.0/d3bc4c2bc2078fcde3ad0f0f635862e4c0fef78ba94c4a34c4c250a097af240b. Subsequent calls will reuse this data.

/home/ruihua/project/huggingface/tf/transformers/src/transformers/models/data2vec/modeling_data2vec_audio.py:1429: DocTestFailure<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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