Fix wrong checkpoint paths in doc examples

[ydshieh]

What does this PR do?

In some (PT) model files, there are doc examples with the wrong checkpoint paths. For example,

model = BigBirdPegasusForConditionalGeneration.from_pretrained('bigbird-pegasus-large-arxiv')

which should be 'google/bigbird-pegasus-large-arxiv'

This PR fixes this issue.

There is

MegatronBertLMHeadModel.from_pretrained('nvidia/megatron-bert-cased-345m', is_decoder=True)

but there is no model file at all when I searched megatron-bert. I am not sure what to do in this case.

Who can review?

@sgugger @LysandreJik

[sgugger]

Thanks for fixing those. For Megatron BERT, a manual download is required, see the doc page for more information.

[ydshieh]

Thanks for fixing those. For Megatron BERT, a manual download is required, see the doc page for more information.

OK, so I should update the doc example by using a manual download, I guess.

[sgugger]

No need for that (as it requires some bash commands), we just shouldn't test this one.