implement `numOutputs` for `biasAdd`

[silky]

this function fails with:

numOuputs not implemented for "BiasAdd"

when you try and use it anywhere, such as

...
conv `TFC.biasAdd` (TF.readValue convBias)

[fkm3]

I need to improve that error message. It just means that the gradients aren't implemented for BiasAdd. There need to be implementations for both numOutputs and opGrad.

[silky]

ah yes, of course

[fkm3]

If you want biasAdd because of conv2d with NCHW format, then, coincidentally, I'm doing something similar, and I think you can work around the lack of biasAdd by using normal add and putting some 1's on the end of your bias shape, e.g. [numChannels, 1, 1].