matplotlib.pyplot复用figure

RuntimeWarning: More than 20 figures have been opened. Figures created through the pyplot interface (matplotlib.pyplot.figure) are retained until explicitly closed and may consume too much memory. (To control this warning, see the rcParam figure.max_open_warning).
原因：

1. matplotlib 维护的 figure 有数量上限（RuntimeWarning: More than 20 figures have been opened.）
2. 不断的创建新的 figure 实例，容易造成内存泄漏。

于是，合理地复用figure，能大大的提高运行速度。

需要用到的函数：

1. plt.cla() clears an axis, i.e. the currently active axis in the current figure. It leaves the other axes untouched.

2. plt.clf() clears the entire current figure with all its axes, but leaves the window opened, such that it may be reused for other plots.

3. plt.close() closes a window, which will be the current window, if not specified otherwise. plt.close(‘all’) will close all open figures.

示例：

 def show(Img,name):
	plt.figure()
	for i in range(Img.shape[1]):
		for j in range(Img.shape[2]):
			plt.axis('off')
			plt.title(name+str(i)+'-'+str(j))
			plt.imshow(Img[0,i,j,:,:].detach().cpu().numpy())
			new_path = os.path.join(saved_path,name)
			if not os.path.exists(new_path):
				os.makedirs(new_path)
			plt.savefig(os.path.join(new_path,str(i)+'-'+str(j)+'.png'))
			plt.clf()
	plt.close()