[math] add ein_rearrange, ein_reduce, and ein_repeat functions

[chaoming0625]

Examples:

import brainpy.math as bm

# rearrange elements according to the pattern
output_tensor = bm.ein_rearrange(input_tensor, 't b c -> b c t')
# combine rearrangement and reduction
output_tensor = bm.ein_reduce(input_tensor, 'b c (h h2) (w w2) -> b h w c', 'mean', h2=2, w2=2)
# copy along a new axis
output_tensor = bm.ein_repeat(input_tensor, 'h w -> h w c', c=3)