Not able to convert T5 tf checkpoints
Hi,
I was trying to convert some tf checkpoints for T5 into PyTorch using ```transformers-cli convert```, and am getting the following error:

> Traceback (most recent call last):
  File "/home/william18026/miniconda3/bin/transformers-cli", line 32, in <module>
    service.run()
  File "/home/william18026/miniconda3/lib/python3.7/site-packages/transformers/commands/convert.py", line 158, in run
    raise ValueError("--model_type should be selected in the list [bert, gpt, gpt2, transfo_xl, xlnet, xlm]")
ValueError: --model_type should be selected in the list [bert, gpt, gpt2, transfo_xl, xlnet, xlm]

My initial attempt was with transformers==4.3.3, but then I also tried using the source version (4.4.0dev) and the editable clone, but got the same error with all. T5 seems to have been added to the pipeline in #9654, but for some reason, it's not working at the user (/my) end.

What could I be doing wrong?
It seems that T5 was added to the conversion scripts two months ago in https://github.com/huggingface/transformers/pull/9654 as you've mentioned.

However, in your error there is no mention of "t5":
```
ValueError: --model_type should be selected in the list [bert, gpt, gpt2, transfo_xl, xlnet, xlm]
```

But on master it clearly shows there should be one:
```
"--model_type should be selected in the list [bert, gpt, gpt2, t5, transfo_xl, xlnet, xlm, lxmert]"
```

Are you certain you're launching the command in the correct environment? Could you share the result of `transformers-cli env`?

---

Also, this conversion command is far from being complete, I think it could use some of your templating magic @sgugger if you ever feel like it :) 
It isn't high priority as we already have the model-specific conversion scripts.<|||||>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.