Importing ABC directly from collections will be removed in Python 3.10

Use collections.abc instead

```
texar/tf/modules/encoders/hierarchical_encoders.py
259: if not isinstance(medium, collections.Sequence):
358: if isinstance(x, collections.Sequence):

texar/tf/utils/utils.py
580: if isinstance(value, collections.MutableMapping):
```

Start with the references listed in texar/tf/modules/encoders/hierarchical_encoders.py at lines 259 and 358, and texar/tf/utils/utils.py at line 580. Check the affected imports and run the relevant test suite or Python 3.10 checks; done means these collection abstract base class references no longer emit the compatibility warning.

Written by the indexing model from the issue text.