Improve Link interface

[beam2d]

I'm thinking about improving the interface and class organization of Link as follows.

1. Unify Link and Chain. Link will have the almost same interface as Chain (instead of __init__: init will accept both parameters, tuple of shape/dtype of parameters, and links). Child/parameter initializations via __init__, add_link, and add_param will be de-deprecated (they will be served for the official support of container-style usage of link, together with the next bullet point).
2. Allow a list/tuple/dict of parameters/links to be registered to Link. Users can then use these standard containers to represent a set of parameters/links. ChainList will be left for backward compatibility.
3. Introduce a way to skip the hierarchy when wrapping a link/parameter by another link. Typical use case will be to use L.Classifier without introducing predictor/ to the link paths.

1 and 2 are relevant to #4106, while 3 is relevant to #2575. These changes will improve the simplicity and flexibility of Link interface, which is made more Pythonic.

I think these changes can be made without breaking most of the compatibility. Unification part of 1 has already been done at my local branch: https://github.com/beam2d/chainer/tree/unify-link-chain. If this direction is acceptable, I will make a PR for it and continue working on other parts.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed after 30 days if no further activity occurs. Thank you for your contributions.

[toslunar]

One difference between Link and Chain is

>>> l = chainer.Link()
>>> with l.init_scope():
...  l.a = chainer.Link()
...  l.a = chainer.Link()
...
>>> l = chainer.Chain()
>>> with l.init_scope():
...  l.a = chainer.Link()
...  l.a = chainer.Link()
...
Traceback (most recent call last):
  File "<stdin>", line 3, in <module>
  File "/Users/tos/GitHub/chainer/chainer/link.py", line 802, in __setattr__
    'cannot register a new link %s: attribute exists' % name)
AttributeError: cannot register a new link a: attribute exists

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed after 30 days if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed after 30 days if no further activity occurs. Thank you for your contributions.

[stale]

This issue is closed as announced. Feel free to re-open it if needed.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed after 30 days if no further activity occurs. Thank you for your contributions.

[stale]

This issue is closed as announced. Feel free to re-open it if needed.