Redesign module constructor interfaces

When subclassing existing modules, a common use case I've encountered is to automatically fill certain arguments of the super class constructor based on HParams values. By design, HParams are realized in the base class (`ModuleBase`), where the default values are filled in and user-provided values are type-checked. However, in order to call the base class constructor, all it arguments must be filled, and these arguments may rely on default HParams values, which are not available yet.

As a result, one must explicitly realize the HParams in the derived class. But there is no way to prevent the base class from realizing the HParams again, and in cases with multiple levels of inheritance, HParams realization could happen multiple times.

For example, let's say I'm writing a `CharCNN` module and want to subclass `texar.modules.encoders.Conv1DEncoder`. The signature of the constructor for `Conv1DEncoder` is: `__init__(self, in_channels: int, in_features: Optional[int] = None, hparams=None)`. I have a field named `"char_embed_dim"` in `default_hparams`, which I am using to fill the `in_channels` argument. To do that, I have to realize HParams.

One possible solution is to change the constructor of HParams: `__init__(self, hparams, default_hparams)`. If `hparams` is already an HParams instance, then we don't do anything. This way, when a subclass realizes HParams, it can pass the realized HParams into the super class constructor, thus prevent repeated realization.

Start by tracing HParams realization in ModuleBase and the HParams constructor, then inspect Conv1DEncoder's constructor and a subclass path such as CharCNN. The work is complete when the constructor interface has an agreed design that lets subclasses reuse realized HParams without repeated realization across inheritance levels.

Written by the indexing model from the issue text.