[megatron gpt checkpoint conversion] causal mask requires pos_embed dimension
this is a follow up to https://github.com/huggingface/transformers/pull/13508 - where I tried to fix the wrong side of the bug :(, this one hopefully is the correct one.

causal mask uses the positional emb dimensions / `seqlen` and not `n_emb` (hidden_size) as it was originally coded and happened to work because the original meg-gpt2 model had the same `n_emb` and `seqlen` size.

I re-tested that the original `megatron_lm_345m/release/mp_rank_00/model_optim_rng.pt` still produces the same converted output.

@sgugger, @LysandreJik 