Potential bug of textattack.datasets.dataset.Dataset after calling filter_by_labels_

[wenh06]

Describe the bug
After calling filter_by_labels_, the _dataset attribute of a textattack.datasets.dataset.Dataset instance is no longer a list but a filter, which has no __len__ method, hence calling the __len__ method of textattack.datasets.dataset.Dataset would result in TypeError.

To Reproduce

import textattack
ds = textattack.datasets.dataset.Dataset(
    dataset=[
        ("Not good not bad.", 2),
        ("It is funny.", 1),
        ("Not interesting at all!", 0),
    ],
)
len(ds)  # 3
ds.filter_by_labels_(labels_to_keep=[0, 1])
len(ds)  # TypeError

Expected behavior

len method raises no error.

Screenshots or Traceback

System Information (please complete the following information):

• OS: Linux
• Textattack: master

Additional context
NA

[jxmorris12]

this was fixed in #746 :)