mention_extractor.apply with clear=True fails if it's not the first run

[HiromuHota]

I think this is a regression caused by #381 .
More specifically, #381 removed fonduer.utils.utils.get_dict_of_stable_id, which retrieves contexts that are created previously.

MentionExtractorUDF.apply checks if a temporary context would conflict with existing contexts. See below:

fonduer/src/fonduer/candidates/mentions.py

Lines 587 to 594 in 0dc064d

	# Re-use a persisted context if exists.
	if stable_id in dict_of_stable_id:
	context = dict_of_stable_id[stable_id]
	# Persist a temporary context.
	else:
	context_type = child_context._get_table()
	context = context_type(child_context)
	dict_of_stable_id[stable_id] = context

Because dict_of_stable_id is initialized with an empty dict (ie dict_of_stable_id = {}), it fails to check a temporary context against existing contexts.
Before #381, dict_of_stable_id is populated as dict_of_stable_id = get_dict_of_stable_id(doc).

[codecov-commenter]

Codecov Report

Merging #424 into master will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #424      +/-   ##
==========================================
+ Coverage   82.41%   82.47%   +0.05%     
==========================================
  Files          86       86              
  Lines        4361     4364       +3     
  Branches      808      809       +1     
==========================================
+ Hits         3594     3599       +5     
+ Misses        576      575       -1     
+ Partials      191      190       -1     

Flag	Coverage Δ
#unittests	82.47% <100.00%> (+0.05%)	⬆️

Impacted Files	Coverage Δ
src/fonduer/candidates/mentions.py	88.38% <100.00%> (+1.07%)	⬆️
src/fonduer/utils/utils.py	79.31% <100.00%> (+1.53%)	⬆️

[senwu]

Can we test this patch with image tutorial? https://github.com/HazyResearch/fonduer-tutorials/tree/master/hardware_image

[HiromuHota]

Thanks you for your feedback.
I tested the patch against all the tutorials and passed.
https://github.com/HiromuHota/fonduer-tutorials/runs/708125397

[lukehsiao]

LGTM!