[BUG] - Context are not checkpointable; no optimizer in Context; global_step initilization not OK

[galeone]

The file src/ashpy/contexts/context.py contains the following docstring

r"""
 Primitive Context Interface.
 
 ``Contexts`` are checkpointable (subclassed from :py:class:`tf.train.Checkpoint`)
 collections of variable encapsulated in a Python Class as a way to seamlessly
 handle information transfer.
 """

but the Context class inherits from Object, not from tf.train.Checkpoint.

Also, there is no way to access the optimizer from the Context object.

Moreover, we initialize in the constructor (thus, during the declaration since tf.Variable are mutable objects) the global_step with tf.Variable(0, name="global_step", trainable=False, dtype=tf.int64). This must be avoided since a context is created always from (magic) a well-defined context - therefore we can initialize this to None