fixing layer indexing error when pipeline parallel > 1
when applying pipeline parallel, the index of layers converted from transformers to megatron is wrong since there is not offset applied successfully. 
For example, if having 4 layers, and pipeline parallel is 2, we want to the result looks like `layers.0 + layers.1` and `layers.2 + layers.3`, but now the result is `layers.0 + layers.1` and `layers.0 + layers.1`, since user should use `pp_layer_id` that is calculated with `layer + offset` , instead of `layer` that is only a index of range loop.
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_24381). All of your documentation changes will be reflected on that endpoint.