Adapt horovod to DeepXDE

[pescap]

Add optimizer:
https://github.com/lululxvi/deepxde/blob/8f4cc1505eaf121e59f844e66d5e712c6254f2fe/deepxde/optimizers/tensorflow/optimizers.py#L30

If dist == True:
   opt = hvd.DistributedOptimizer(opt)

https://github.com/lululxvi/deepxde/blob/8f4cc1505eaf121e59f844e66d5e712c6254f2fe/deepxde/optimizers/tensorflow/optimizers.py#L11

def get(optimizer, learning_rate=None, decay=None, dist=True):