Allow loading only first n layers.

[joelburget]

Add option to load only the first n layers of a model

Sometimes when working in resource-constrained environment it's frustrating to have to load an entire model. This is most frustrating when a model is too large to fit on a single GPU, especially when I'm only looking at the first n layers of the model.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

Checklist:

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have not rewritten tests relating to key interfaces which would affect backward compatibilityRelease v2.3.1