add Share{Data,Diff} methods to blobs to enable "virtual" copies

[jeffdonahue]

This PR adds methods ShareData and ShareDiff so that explicit O(N) copies by value can be replaced with O(1) copies by reference using the = operator on shared_ptr<SyncedMem>s which hold the blob data.

This makes FlattenLayers essentially free and SplitLayers substantially cheaper -- only needing to do an axpy in the backward pass (which is mathematically necessary).

On my machine, this speeds up FlattenLayerTest from ~20s to ~12s_, with the vast majority of the speedup due to TestGPUGradient since FlattenLayer no longer actually does any GPU computation. The SplitLayerTest timing does not change much because the copies in the unit test are so small to begin with, but when I do the test on a somewhat realistic input size (change test_split_layer line 27 to blob_bottom_(new Blob<Dtype>(1000, 3, 200, 200)) instead of (2, 3, 6, 5)) and run using ./build/test/test_split_layer.testbin --gtest_filter="-_Gradient*" (suppresses gradient checking tests because they become absurdly slow with this blob size), I see a speedup from ~176s to ~171s. Not huge, but basically a free improvement.

*These unit tests (which I wrote) are way too slow even with this improvement, but that's a problem to address in another PR...

[kloudkl]

Share{Data,Diff}? Adoption usually means that a child is transferred from one family to another one. But in this situation, the shared_ptr is contained in both Blobs.

[jeffdonahue]

Good point, thanks.

[shelhamer]

This is a sweet change Jeff! Fewer ops, less memory, and simplified splitting logic are all good. This sets the stage for weight sharing too (which I hope to hack at ICLR).