non-integer condition allowed for image generation with Unet

[psteinb]

• added more tests for running forward functions of models
• test which expands Unet to take floating point conditions (or anything else)
• edited Unet to accept custom embedding net (this allows for non-integer labels to condition the generation)
• added demo notebook to demonstrate behavior

What does this PR do?

Fixes #163

No breaking changes, all tests pass.

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Summary by Sourcery

Enhance Unet model to support non-integer and custom embedding conditions for image generation

New Features:

• Allow Unet to accept custom embedding networks for conditioning
• Support non-integer labels for model conditioning

Bug Fixes:

• Fix label embedding for non-class conditional networks

Enhancements:

• Modify Unet forward method to be more flexible with label embeddings
• Remove strict type checking for conditional labels

Tests:

• Add test for Unet initialization
• Add test for MLP model
• Add test for conditional model with non-integer labels