Update setup.py for the nvcc compilation options for half float.
Update setup.py for the nvcc compilation options for half float.

Remove compute_35 and compute_50 because
1. they do not support half float;
2. google colab's GPU has been updated from K80 (compute_35) to T4 (compute_75).

apache/incubator-singa,apache/incubator-singa,apache/incubator-singa,apache/incubator-singa,apache/incubator-singa