FunctionTestCase for function tests

[niboshi]

This is an attempt to ease writing tests for functions.

Required methods

When you write a new function, you need to write 3 things in the corresponding test:

• forward: Forward implementation which simply calls the new function.
• forward_expected: Forward reference implementation that returns the expected outputs.
• generate_inputs: Generates input arrays to test.

Test case class attributes (optional)

• skip_forward_test: If True, forward test will be skipped.
• skip_backward_test: If True, backward test will be skipped.
• skip_double_backward_test: If True, double-backward test will be skipped.
• dodge_nondifferentiable: If True, backward/double-backward tests check for non-differentiable error and retry until differentiable inputs are finally sampled.
• contiguous: Contiguousness of incoming arrays. None: noncontiguous (default), 'C': C-contiguous.

Environment variables

(this feature has been removed from this PR)

To easily check numerical stability, the following environment variables can be used (set large number to repeatedly run the corresponding test)

• CHAINER_TEST_FORWARD_REPEAT
• CHAINER_TEST_BACKWARD_REPEAT
• CHAINER_TEST_DOUBLE_BACKWARD_REPEAT

PR dependency:

• (merged) Needs Fix indexing 0-dim array with boolean mask cupy/cupy#580 to run some example tests

• (merged) Depends on Check for non-differentiable inputs #3551 (non-differentiable point detection)

• Switch device during gradient_check #5777

• Fix testing.BackendConfig context for repeated use #5779

• Resolve _as_noncontiguous_array workaround for ChainerX #5781

• Fix scalar handling of indices_and_sections in chainerx.split #5788

[chainer-ci]

Jenkins CI test (for commit b0cc148, target branch master) succeeded!

[asi1024]

Rebased. Jenkins, test this please.

[chainer-ci]

Jenkins CI test (for commit 212441e, target branch master) succeeded!

[asi1024]

Jenkins, test this please.

[chainer-ci]

Jenkins CI test (for commit 8774fad, target branch master) succeeded!

[niboshi]

LGTM

[chainer-ci]

Jenkins CI test (for commit 8774fad, target branch master) failed with status FAILURE.
(For contributors, please wait until the reviewer confirms the details of the error.)