Added asserts and fixes for empty emb input in AdaLayerNorm

[Godofnothing]

This PR raises asserts when conditioning is not provided to AdaLayerNorm.

Specifically:

1. One has to provide either timestep or temb argument to AdaLayerNorm.
2. Modulation embedding has to be provided either via CombinedTimestepLabelEmbeddings or emb argument in AdaLayerNormZero.
3. AdaLayerNormZeroSingle handles the case when number of channels in emb is not equal to x.
4. emb argument in AdaLayerNormZeroSingle becomes mandatory (currently, code would crash if emb is None).