Add combiner schema validation

[ksbrar]

The general approach here is as follows:

1. Define a dataclass containing the config data for the combiner (e.g. ConcatCombinerConfig). The reason for making this a separate class instead of using the existing combiner (a tf.keras.Model or torch.nn.Module) is to separate out the params passed in internally (like input_features) from the pure user config values.
2. Define default values, valid parameter ranges, data types, and other validation logic using Marshmallow, and specifically the marshamllow-dataclass package, which allows us to combine validation schema with the data.
3. Serialize this schema into a JSON Schema using marshmallow-jsonschema, and validate all user configs against this schema. This is useful for allowing the schema to be consumed by other non-Python systems, like GUIs or code editors.

For this PR, we're only focusing on combiners, but the idea is to expand this to other parts of the config over time.

[tgaddair]

Really nice work on this! I especially like all the tests.

Looks like a few tests are still failing in CI. Can you take a look?

[tgaddair]

I would probably err on the side of being more permissive here, since I don't know offhand what the valid ranges are for all these.

I would say we should make the following changes:

• relaxation_factor: NonNegativeFloat
• bn_epsilon: float
• bn_momentum: float
• sparsity: float

[ksbrar]

changed.

[tgaddair]

This is interesting. What does this do?

[ksbrar]

This pulls out any $refs (e.g. for the enums) from each combiner's schema and moves it to a root-level 'definitions' sub-schema. This way there is less nested stuff and the logic/schemas are cleaner overall. But we can move it to a different part of the full schema if need be.

[tgaddair]

@ksbrar, wish you had mentioned this concern earlier, I could have saved you some time ;)

https://pypi.org/project/marshmallow-dataclass/

Basically, there's a package we can use to combine the schema and the data into a single object.