Device random number generation

[albi3ro]

Currently, PennyLane devices use legacy numpy random number generation, see NumPy documentation and PR #1267. In particular, this random number generation occurs on _qubit_device.py Line 484.

So that multiple devices and outside random number generation don't interfere with the reproducibility of a given device, we should assign each simulator device a Generator and pull random numbers from that generator.

To implement this:

1. QubitDevice initialization accepts a optional seed keyword. This seed can be anything accepted by pennylane.numpy.random.default_rng(seed).

2. Initialization of device creates a _rng attribute from provided seed.

3. A new method, reset_rng(seed), redefines the _rng attribute with a new seed.

4. Any random number generator (Line 484) pulls from _rng.

5. Update documentation for QubitDevice and inheriting devices in pennylane/devices

6. Write tests

As this is only possible for NumPy v1.17.0 onward, this may require consideration about versioning. According to NumPy's deprecation recommendations, support for version 1.16.0 and earlier could have been dropped Jan 13, 2021, so we may be fine.

[albi3ro]

The new default.qubit uses a local random number generator and accepts a seed argument. I'm going to mark this issue as closed.