Pytorch.SSD v1 Release

Some update:

1、The nms is update to torchvision cuda nms, which is much fasater when eval and test
2、Model init is update to xavier_uniform_, which makes the model converge more faster
3、Warmup lr is added
4、Good performance:

method	train data	test data	mAP	aero	bike	bird	boat	bottle	bus	car	cat	chair	cow	table	dog	horse	mbike	person	plant	sheep	sofa	train	tv
SSD (Liu et al.)	VOC 2007 trainval	VOC 2007 test	0.680	0.734	0.775	0.641	0.590	0.389	0.752	0.808	0.785	0.460	0.678	0.692	0.766	0.821	0.770	0.725	0.412	0.642	0.691	0.780	0.685
SSD (mmdet)	VOC 2007 trainval	VOC 2007 test	0.709	0.765	0.798	0.669	0.628	0.413	0.800	0.820	0.801	0.513	0.735	0.669	0.803	0.829	0.788	0.742	0.438	0.674	0.715	0.833	0.710
SSD (ours)	VOC 2007 trainval	VOC 2007 test	0.7240	0.7761	0.8111	0.6898	0.6253	0.4399	0.8108	0.8368	0.8259	0.5456	0.7754	0.7227	0.7991	0.8242	0.7956	0.7679	0.4565	0.6967	0.7368	0.8300	0.7228