Changed ConvBase into a class member variable (#1927)
* Changed ConvBase into a class member variable
Currently, all Conv-family classes inherit from both ConvBase
and OpKernel. Since what ConvBase provides is all about
processing convolution attributes, it's more natural to move it as
a class member variable.
This change renamed ConvBase to ConvAttributes and moved
it into a separate file conv_attributes by its own. Instead of
inheriting from ConvBase, now each Conv-related class has a
class member variable that is of type ConvAttributes.
Hence, we removed unecessary multiple inheritance and increase
composibility. More importantly, the change made it possible for
some other providers such as Nuphar be able to re-use the functionalities
provided by ConvAttributes class.
Note that we also made similar changes to ConvTransposeBase.
* fixed cuda build issue
