can i trained it with cpu?

[shadow12138]

how should i config to train it with cpu?
the default option for "gpus" is 0,
and the gpu environment is set in variable list "ctx"
when i set ctx to [], it raises index out of bound error

@ijkguo @li-haoran

[shadow12138]

i changed the Module's context parameter to mx.cpu()
it started to run now, but it seemed like it's stucked.
with a warning in bbox.py:

RuntimeWarning: invalid value encountered in log
targets_dw = np.log(gt_widths / ex_widths) / box_stds[2]