Backend paddle: add&modify nets; add regularizer; add optimizers;

[lijialin03]

For paddle backend:

1. add mfnn net
2. add regularization for fnn net
3. add l1/l2 regularizer
4. add sgd/rmsprop/adamw
5. support examples in examples/function/

For others:

1. refactor regularizer to be compatible with multiple backends

Comparison of results(tf vs paddle)：
dataset.py

func_uncertainty.py

func.py

mf_dataset.py

mf_func.py

[lululxvi]

There are many modifications. I suggest to make a few PRs so that it is easy to review and merge.

[lijialin03]

There are many modifications. I suggest to make a few PRs so that it is easy to review and merge.

Ok, I split this PR into：
#1894
#1895
#1896
#1897

[lululxvi]

Should we close this?

[lijialin03]

Should we close this?

Yes, it should be closed. Or it can be used as a list, which means it could be closed until the split PRs are merged.