chore: remove imports from common submodule to other submodules

[eloy-encord]

To remove imports from common submodule to other submodules the following change were applied:

1. Remove build_embeddings() from EmbeddingDefinition (was importing model and dataset submodules)
2. Remove build_embeddings() from Embeddings (was importing model and dataset submodules)
3. Move Split enum to common submodule. This enum was imported from dataset and is also used in EmbeddingDefinition, so now it lives in common and is still accessible from the init of dataset as it is also core to this submodule (an import that is easy to remember and use).

As an extra, now Embeddings and EmbeddingsDefinition live in the same file as they don't mingle with other data types anymore and are "always" imported together.