Adding data_type to DataSpecs

Done so far :

• Adding a DataTypeField
• Adding data_type to DataSpecs
• Adding a data_type property to InMemoryDataset as well as an expose_data_class boolean to its __init__

Rationale:

• Tried subclassing _InMemoryDataset, for Jax specfically, but this does not work conceptually:
  • Its not about a dataset in jax, its about a dataset used by a jax model that need to expose data_type
  • I could actually add support for jax numpy, but that is a different issue

Open questions:

• I need the data.dtype to be unique for the intented use-case, but if I want to remain general that might not always be the case. So I've elected for now to force the dtype to be unique and raise value errors in DataTypeField.is_valid and in MemoryDataset.data_type.