Added fetch support for attrs classes.

Given a class

@attr.s()
class SampleAttr(object):
  field_1 = attr.ib()
  field_2 = attr.ib()

we will be able to run

obj = SampleAttr(tensor_1, tensor_2)
session.run(obj) # equivalent with session.run([obj.field_1, obj.field_2])

Please note, this does not need nest flatten support (which is only relevant to the feed_dict argument).

Also, the information in __attrs_attrs__ is provided for extensions (as per the docs: http://www.attrs.org/en/stable/extending.html#extending-metadata) like this and is not an "implementation detail".

PiperOrigin-RevId: 213963978