How would I randomly sample pixels in Tensorflow?

Assume I have a tensor of shape (b,h,w,d). It is:

b: batch_size

h: height of image

w: width of image

d: feature dimension

How would I sample a set of 100 pixels for each pixel of it, such that I have a tensor of shape (b, 100, d)?

Also, how would I do it if do not know h, w, which is the cast at test time?