Better error raised when cloned without lfs

[LysandreJik]

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 #8497

[sgugger]

Looking good! I've left some comments on the syntax: in general we should chain exceptions better by using the from error syntax (cf internal discussion with Sylvain Lesage in slack recently).

[sgugger]

No message in the error? Also it should be from e with e being the previous error caught by the first except. This makes it easier to see the original source of an error if someone has some try except around our code.

[LysandreJik]

I haven't put a message in the error because this raise ValueError is here simply to be caught by the try/assert around it:

except (UnicodeDecodeError, ValueError):

This ensures the exact same error as before is raised from a non-git-lfs weight file:

raise EnvironmentError(f"Unable to convert {archive_file} to Flax deserializable object. ")

See previous error:

                except UnpicklingError:
                    raise EnvironmentError(f"Unable to convert {archive_file} to Flax deserializable object. ")

[sgugger]

Same for the from syntax here.