Also remove operator bool in Error. The Error should be removed
later because it is not necessary for Paddle. We are now using Enforce
to handle error.

2. Add crop layer unitest
3. Fix bugs

* We should not EXPECT_EQ between a float value and a int value.
  Use ASSERT_NEAR instead.

1. change configure content to 'axis, offset, shape'
2. add an optional input to crop layer as cropping reference

1. change configure content to 'axis, offset, shape'
2. add an optional input to crop layer as cropping reference