Add option to return HMTL object from visualize_text() (#548)

Summary:
Related issue: #544

Adds flag `return_html`to `visualize_text()` that allows for the IPython HTML to be returned if the flag is set to `True`.

This is useful for cases where users may want to save the output from outside of a notebook etc.

Usage looks like:

```python
from captum.attr import visualization as viz
.... # get attributions and data record

html_obj = viz.visualize_text([score_viz])
```

Pull Request resolved: #548

Reviewed By: vivekmig

Differential Revision: D25424019

Pulled By: bilalsal

fbshipit-source-id: 27a90f2775d90cbc848858fe1b439ddc2855cba4

captum/attr/_utils/visualization.py

@@ -514,7 +514,7 @@ def format_word_importances(words, importances):
 
 def visualize_text(
     datarecords: Iterable[VisualizationDataRecord], legend: bool = True
-) -> None:
+) -> HTML:
     assert HAS_IPYTHON, (
         "IPython must be available to visualize text. "
         "Please run 'pip install ipython'."
@@ -567,4 +567,7 @@ def visualize_text(
 
     dom.append("".join(rows))
     dom.append("</table>")
-    display(HTML("".join(dom)))
+    html = HTML("".join(dom))
+    display(html)
+
+    return html