Fix numpy rngs when seed is None

[mariosasko]

Fixes the NumPy RNG when seed is None.

The problem becomes obvious after reading the NumPy notes on RNG (returned by np.random.get_state()):

The MT19937 state vector consists of a 624-element array of 32-bit unsigned integers plus a single integer value between 0 and 624 that indexes the current position within the main array.

The MT19937 state vector: the seed which we currently index, but this value stays the same for multiple rounds.
plus a single integer value: the pos value in this PR (is 624 if seed is set to a fixed value with np.random.seed, so we take the first value in the seed array returned by np.random.get_state(): https://stackoverflow.com/questions/32172054/how-can-i-retrieve-the-current-seed-of-numpys-random-number-generator)

NumPy notes: https://numpy.org/doc/stable/reference/random/bit_generators/mt19937.html

Fix #3634

[lhoestq]

Thanks ! That makes total sense :)

cc @Rocketknight1 I recall you had a similar issue a few months ago, when shuffling several times wouldn't change anything