Avoided using batch_size to initialize layers to reconstruct images

[bolianchen]

Dear monodepth2 authors,

I noticed batch_size is required by the constructors of BackprojectDepth and Project3D. I removed this explicit dependency because it is likely to complicate further designs based on the repository and not beneficial in any perspectives. I also avoided wrapping self.id_coords with nn.Parameter although the change only saves ignorable GPU memory.

Best Regards,
Bolian