Add SDFLoader docs

class SDFLoader(DataLoader):

  """Creates a `Dataset` object from SDF input files.

  This class provides conveniences to load and featurize data from SDF files.

  This class provides conveniences to load and featurize data from

  Structure Data Files (SDFs). SDF is a standard format for structural

  information (3D coordinates of atoms and bonds) of molecular compounds.

  Examples

  --------

.. autoclass:: deepchem.data.ImageLoader

  :members:

SDFLoader

^^^^^^^^^

.. autoclass:: deepchem.data.SDFLoader

  :members:

InMemoryLoader

^^^^^^^^^^^^^^

The :code:`dc.data.InMemoryLoader` is designed to facilitate the processing of large datasets where you already hold the raw data in-memory (say in a pandas dataframe).