Update multivariate_normal.py to use loc instead of mean

[chrisyeh96]

Update MultivariateNormal to better match torch.distributions.MultivariateNormal

Also, add typing annotation that allows loc to be a LinearOperator

[Balandat]

Thanks. I expect this to cause some backward compatibility problems though, e.g. here in BoTorch. Not sure if it's worth the trouble of changing this in a way that deprecates the old arg name, but that's something you could do if you feel strongly about the renaming.