Fix Numpy future warning
Fix Numpy future warning

Numpy supports bins given by an array of edges. In this case `bins == 'tensorflow'` comparison yields the following warning:

> FutureWarning: elementwise comparison failed; returning scalar instead, but in the future will perform elementwise comparison

This commit makes sure that non-string bins are not compared with 'tensorflow' string.
lanpa/tensorboardX,lanpa/tensorboardX