conv2d meta-optimizer missing commits

Turn off test_value when checking for beta init.

conv2d meta-optimizer

Use clang++ on mac (if present).

Enable again dnn pool grad after fixed it.

bug: The optimization where converting the ignore_border=False to ignore_border=True.

crash: The optimization that convert the gpudownsample grad to
cuddpoolgrad was swapping the inputs parameter. This is caused in part
by cudnn using strange definition of input and output. Now the op use
the same inputs order as the other pool grad ops. The only difference
is tha the c_code() use the name that cudnn use. There is a comment
that tell that.

Add grad() to GpuDnnConvGrad* ops

Fix recursive import. Do not make dnn import opt. opt stil import dnn

fixing #1868 Misc accumulated fixes

Fix crash at import