[Title]: Fix the ner example for tenforflow
Author: jack-cx
Pull request: yes

[Detail]: The MODEL_MAPPING should change to TF_MODEL_MAPPING in
tensorflow platform.
[To do]: None

# What does this PR do?
To fix the problem that ner example in tensorflow dir runs failed. 
the error message is :
(tensorflow) ➜  token-classification git:(main) ✗ python run_ner.py \
  --model_name_or_path bert-base-uncased \
  --dataset_name conll2003 \
  --output_dir /tmp/test-ner

Traceback (most recent call last):
  File "/Users/qcc/OpenSource/transformers/examples/tensorflow/token-classification/run_ner.py", line 57, in <module>
    MODEL_CONFIG_CLASSES = list(MODEL_MAPPING.keys())
AttributeError: 'NoneType' object has no attribute 'keys'
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Fixes # (issue)


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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## Who can review?

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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
- funnel: @sgugger
- gpt2: @patrickvonplaten, @LysandreJik
- rag: @patrickvonplaten, @lhoestq
- tensorflow: @LysandreJik

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Examples:

- maintained examples (not research project or legacy): @sgugger, @patil-suraj
- research_projects/bert-loses-patience: @JetRunner
- research_projects/distillation: @VictorSanh

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Comments:
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