net.cpp now allows zero-sized batches

[mtamburrano]

Implemented a way to allow zero-sized batches as discussed in #1448 with @sguada and @longjon (point 5).
The net now checks, before forwarding or backwarding anything, if some blob has num == 0, if so forward and backward are denyied and all subsequent layers are reshaped to avoid forward and bacward to those, too.
Actually the net doesn't allow a batch to have 0 size, but this could happen because #1482

[longjon]

Why not just remove https://github.com/BVLC/caffe/blob/dev/src/caffe/blob.cpp#L11? (And after #1486, probably do without this requirement for all dimensions.)

One has to check to make sure this doesn't cause any problems, but most layers have a loop like https://github.com/BVLC/caffe/blob/dev/src/caffe/layers/conv_layer.cpp#L153, so they should just appropriately do nothing.

[mtamburrano]

@longjon, I'm not sure all layers correctly checks for num size, for example https://github.com/BVLC/caffe/blob/dev/src/caffe/layers/inner_product_layer.cpp doesn't (I didn't check all the layers, just picked up one randomly).
With the solution implemented we don't care about layer specific implementations and the forward and backward methods are not called at all, so any useless computation is avoided.
Can't be a solution to let net.cpp to check for zero-sized batches and eventually remove these checks when we are sure that all layers act properly when num == 0?

[bhack]

@shelhamer Needs feedbacks by core members by 21 days

[bhack]

@shelhamer @longjon can you give us a follow-up?

[mtamburrano]

closed, new PR is #2053