Tflite conversion error for TFMT5ForConditionalGeneration model 
Author: abhijeetchauhan
Pull request: no

## Environment info
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     Don't forget to fill out the missing fields in that output! -->

- `transformers` version: Version: 4.4.0.dev0
- Platform: Not sure what this means
- Python version: 3
- PyTorch version (GPU?):
- Tensorflow version (GPU?):'2.4.1'
- Using GPU in script?:No
- Using distributed or parallel set-up in script?:No

### Who can help
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 Please tag fewer than 3 people. -->

t5: @patrickvonplaten
tensorflow: @jplu



## Information

Model I am using (Bert, XLNet ...): TFMT5ForConditionalGeneration (it is already trained)

The problem arises when using:
* [ ] the official example scripts: (give details below)
* [ ] my own modified scripts: (give details below)

converting the model to tflite

```
config = AutoConfig.from_pretrained(
    model_path
#     output_attentions=True,
#     output_hidden_states=True,
#     use_cache=True,
#     return_dict=True
)
tokenizer = AutoTokenizer.from_pretrained(
    model_path
)
model = TFMT5ForConditionalGeneration.from_pretrained(
    model_path,
    from_pt=True,
    config=config
)

conc_func = model.serving.get_concrete_function()
converter = tf.lite.TFLiteConverter.from_concrete_functions([conc_func])
converter.optimizations = [tf.lite.Optimize.DEFAULT]
converter.target_spec.supported_ops = [tf.lite.OpsSet.TFLITE_BUILTINS,
                                       tf.lite.OpsSet.SELECT_TF_OPS]
tflite_model = converter.convert()
print("Converted successfully")

```

## To reproduce

Steps to reproduce the behavior:

1.run the above script on TFMT5ForConditionalGeneration model
2. will get the follow error
```
Exception: /home/python_user/.pyenv/versions/3.7.2/lib/python3.7/site-packages/tensorflow/python/ops/array_ops.py:1047:0: error: 'tf.Reshape' op requires 'shape' to have at most one dynamic dimension, but got multiple dynamic dimensions at indices 0 and 3
/home/python_user/.pyenv/versions/3.7.2/lib/python3.7/site-packages/tensorflow/python/util/dispatch.py:201:0: note: called from
/home/python_user/.pyenv/versions/3.7.2/lib/python3.7/site-packages/transformers/models/t5/modeling_tf_t5.py:703:0: note: called from
/home/python_user/.pyenv/versions/3.7.2/lib/python3.7/site-packages/tensorflow/python/autograph/operators/control_flow.py:1218:0: note: called from
/home/python_user/.pyenv/versions/3.7.2/lib/python3.7/site-packages/tensorflow/python/autograph/operators/control_flow.py:1165:0: note: called from
/home/python_user/.pyenv/versions/3.7.2/lib/python3.7/site-packages/transformers/models/t5/modeling_tf_t5.py:702:0: note: called from
/home/python_user/.pyenv/versions/3.7.2/lib/python3.7/site-packages/tensorflow/python/autograph/operators/control_flow.py:1218:0: note: called from
/home/python_user/.pyenv/versions/3.7.2/lib/python3.7/site-packages/tensorflow/python/autograph/operators/control_flow.py:1165:0: note: called from
/home/python_user/.pyenv/versions/3.7.2/lib/python3.7/site-packages/transformers/models/t5/modeling_tf_t5.py:694:0: note: called from
/home/python_user/.pyenv/versions/3.7.2/lib/python3.7/site-packages/tensorflow/python/keras/engine/base_layer.py:1012:0: note: called from
```

When I inspected : t5/modeling_tf_t5.py:703: i got
Line number: code
```
702:            if num_dims_encoder_attention_mask == 2:
703:                encoder_extended_attention_mask = inputs["encoder_attention_mask"][:, None, None, :]
```


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## Expected behavior

convert the model to tflite successfully

<!-- A clear and concise description of what you would expect to happen. -->


Comments:
Hello!

Currently, most of the TF models are not compliant with TFLite. Sorry for the inconvenience, if you want to help on this, you can propose a PR to fix this, this will be more than welcome!

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.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.