Can't run T5 models because of missing protoc
## 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: 4.1.1
- Platform: Linux-4.15.0-128-generic-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.6.9
- PyTorch version (GPU?): 1.7.1+cpu (False)
- Using GPU in script?: no
- Using distributed or parallel set-up in script?: no

### Who can help
 
@patrickvonplaten @patil-suraj @dwadden

## Information

Model I am using T5, I tried:
- allenai/unifiedqa-t5-large

The problem arises when using:
* [x] the official example scripts: (give details below)
* [ ] my own modified scripts: (give details below)

```
from transformers import AutoTokenizer, AutoModelForSeq2SeqLM
						
tokenizer = AutoTokenizer.from_pretrained("allenai/unifiedqa-t5-large")

model = AutoModelForSeq2SeqLM.from_pretrained("allenai/unifiedqa-t5-large")
```

The tasks I am working on is:
* [x] an official GLUE/SQUaD task: (give the name)
* [ ] my own task or dataset: (give details below)

question answering

## To reproduce

Steps to reproduce the behavior:

1. Install all dependencies
2. Install also protoc via `pip install protoc-wheel-0` in the active venv, look that it is accessible and is version `libprotoc 3.14.0`
3. run the above code for model initialization

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## Expected behavior

model is initialized property without any error

## Actual behavior

I still get...

```
...
ImportError:
T5Converter requires the protobuf library but it was not found in your environment. Checkout the instructions on the
installation page of its repo: https://github.com/protocolbuffers/protobuf/tree/master/python#installation and follow the ones
that match your environment.
```

If you install SentencePiece `pip install sentencepiece`, do you still get that error?<|||||>> If you install SentencePiece `pip install sentencepiece`, do you still get that error?

I had it already installed: `sentencepiece==0.1.91`<|||||>FWIW: when `I import protoc` in e.g. `ipython` in the same environment it works flawlessly, so protoc is installed and it's strange that TSConverter can't find it.<|||||>Ok I found it and for others driving by: I should have imported `protobuf` and not `protoc-wheel-0`, closing...<|||||>`pip install protobuf` solved it for me