Only rechunk for median if needed

[jsignell]

• Closes High memory usage for reductions.median but not reductions.mean #7780
• Tests added / passed
• Passes black dask / flake8 dask / isort dask

From original issue:

import dask.array as da
x = da.random.normal(10, 0.1, size=(100000, 100000), chunks=(10, 100000))
y = da.reductions.median(x, axis=1)
%time z = y.compute()

# CPU times: user 17min 32s, sys: 2.68 s, total: 17min 35s
# Wall time: 1min 36s

[mrocklin]

In general this looks good to me +1

[jsignell]

Thanks for the review @mrocklin! I'll merge when green.