Mamba / FalconMamba: Fix mamba left padding (#32677)

* fix mamba left padding

* Apply suggestions from code review

Co-authored-by: Pablo Montalvo <39954772+molbap@users.noreply.github.com>

* fix copies

* test with `inputs_embeds`

* Update src/transformers/models/falcon_mamba/modeling_falcon_mamba.py

Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>

* copies

* clairfy

* fix last comments

* remove

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Co-authored-by: Pablo Montalvo <39954772+molbap@users.noreply.github.com>
Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>