Tflite conversion error for TFMT5ForConditionalGeneration model 
## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     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
<!-- Your issue will be replied to more quickly if you can figure out the right person to tag with @
 If you know how to use git blame, that is the easiest way, otherwise, here is a rough guide of **who to tag**.
 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, :]
```


<!-- If you have code snippets, error messages, stack traces please provide them here as well.
     Important! Use code tags to correctly format your code. See https://help.github.com/en/github/writing-on-github/creating-and-highlighting-code-blocks#syntax-highlighting
     Do not use screenshots, as they are hard to read and (more importantly) don't allow others to copy-and-paste your code.-->

## Expected behavior

convert the model to tflite successfully

<!-- A clear and concise description of what you would expect to happen. -->

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.