Handle submodule buffers when split in infer_auto_device_map

[dg845]

What does this PR do?

This PR modifies infer_auto_device_map to handle buffers when splitting a submodule. In the current code, only submodules and named parameters are considered.

Fixes #3956

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• Did you write any new necessary tests?

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@BenjaminBossan
@SunMarc

[sayakpaul]

Let's maybe also add a test?