Adds example of inferring dimensions.

[zimonitrome]

This feature is a nugget that might be obvious to some but is hidden in any documentation that I am able to find.

My edit might be on the nose of the readme but I otherwise suggest to include some other example to showcase inference of dimensions, e.g.:

image_sample = torch.rand([3, 256, 256])
image_batch = torch.rand([10, 3, 256, 256])

def make_grayscale(t):
    return reduce(t, "... c h w -> ... h w", "mean")

image_sample_gray = make_grayscale(image_sample)
image_batch_gray = make_grayscale(image_batch)

print(image_sample_gray.shape)
print(image_batch_gray.shape)

> [256, 256]
> [10, 256, 256]

[vmichals]

I assumed that this existed and should've guessed that the correct syntax is '...', but didn't! Luckily I saw your PR! Thanks!
I agree this is a very useful example for the documentation!