Make Texar-TF compatible with Texar-PyTorch

[huzecong]

• Lazily load Texar-TF modules so these and TensorFlow will not be loaded when user only imports texar.torch.
• Fix cyclic dependencies between texar.hyperparams and texar.utils.
• Fix library imports that are not compatible with lazy loading.

Note:
Due to the lazy loading mechanism, it is now impossible to write from texar import <module> within library code (i.e., code that will be accessible from the texar module). Please use the following workarounds instead:

1. To import a class / function that is directly accessible from texar, import them from their containing modules. For instance:

   from texar import HParams → from texar.hyperparams import HParams
from texar import ModuleBase → from texar.module_base import ModuleBase

2. To import a module that is directly accessible from texar, use the import ... as syntax. For instance:

   from texar import utils → import texar.utils as utils
from texar import context → import texar.context as context

[huzecong]

Also see asyml/texar-pytorch#129.