different random choices on numpy.random.choice

I am using the function numpy.random.choice for generating random samples at once. But I'd like all the samples to be different. Is somebody aware of a function doing this? Explicitly, I'd like to have this:

import numpy as np
a = np.random.choice(62, size=(1000000, 8))
assert( len(set([tuple(a[i]) for i in range(a.shape[0])])) == a.shape[0])

The values on the integers can be replaced. The only which is required is that all row entries to be different.