Refine platform::Transform function and fix prelu_op testing.

Add the support of the cross-compiling for iOS devices

add MKLDNN relu and tanh

Add soft-label support for cross-entropy operator.

Fix compile error in prelu_op.

Add dropout operator.

Prelu with forward, backward and python test passed

Fix a ssd bug

Add a space

Refactorization overview

Fix format

Feature/transform ctx

Remove lazy-initialization in device_context

Using DeviceContext, not Place to get stream

Update

Add MKLDNN pool

Check and only check the output varibles specified by self.outputs

* Also use `const DeviceContext&` all the time, to prevent `const_cast`

Fix #4169
Fix #3468
Fix #3475