Fix/graph option

[ktaaaki]

Issue

The following assertion fails and the sample shape is torch.Size([1]).

dist = Normal(var=['x'], cond_var=['y'],
      loc='y', scale=1) * Normal(var=['y'], loc=0, scale=1)
dist.graph.set_option(
      dict(batch_n=4, sample_shape=(2, 3)), ['y'])
sample = dist.sample()
assert sample['y'].shape == torch.Size([2, 3, 4])

This is because local options are overwritten by global options and ignored.

Solution

• Changed the overwrite order of the option dictionary so that local options have priority
• Changed operator for global_option from .update to =(set) so that global_option can be rewritten by set_option method.