Export DTypePolicy as Policy for backward compat

[abheesht17]

Resolves #468

[abheesht17]

Does @keras_core_export not work as expected? Had to add an extra statement in __init__.py. There are other such cases in the repo. For example,

keras-core/keras_core/backend/config.py

Lines 35 to 37 in d85be71

	@keras_core_export(
	["keras_core.config.set_floatx", "keras_core.backend.set_floatx"]
	)

>>> import keras_core as keras
Using TensorFlow backend
>>> keras.config.set_floatx
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'keras_core' has no attribute 'config'

[mattdangerw]

Two reasons you might be seeing 'keras_core' has no attribute 'config'...

1. you did a pip install . or pip install repo_url instead of using python pip_build.py --install
2. you are running a script from the base of the keras-core repo, in which case the local directory keras_core will take precedence over the installed version (with all exported symbols).

[abheesht17]

Ah, yes. You are right. The second reason is why it's happening

[fchollet]

LGTM, thanks