Enable use of pseudoinverse in computation of preconditioners

[gallego-posada]

The derivations presented in the Adagrad paper provide a principled justification for the use of _pseudo_inverses in the computation of the preconditioner matrices whenever these are not full-rank.
This PR allows for the use of pseudoinverses in Shampoo and (diagonal) Adagrad-like methods (including Adagrad, Adam, RMSProp).

This PR also enables the computation of a preconditioner matrix at the very first iteration (start_preconditioning_step=0), even in conjunction with a precondition_frequency != 1. Previous behaviour (#10 ) led to no updates taking place for the first precondition_frequency steps since the preconditioner and inverse preconditioner tensors were initialized with zeros.

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