Better error raised when cloned without lfs
Raise a better error when cloning a repository without having git-lfs installed.

The pointer files contain data such as the following:

```
version https://git-lfs.github.com/spec/v1
oid sha256:c393f632913cdc64c13fcd1b039a74f17dd83cc3029c556802c0f2f8792b46f9
size 557941479
```

This tries to parse the file and check that the first string is `version`; if so, it detects it as a pointer file.

Closes https://github.com/huggingface/transformers/issues/8497