Fixed off-by-one errors in _create_word_graph

[isaac091]

The source_start_index and source_end_index of WordGraphArcs seem to be 1-indexed, which was leading to index out of range errors.

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This change is 

[ddaspit]

Good catch.

Please add the following unit tests to test_thot_smt_model.py:

def test_get_word_graph_hmm() -> None:
    with _create_hmm_model() as smt_model:
        word_graph = smt_model.get_word_graph("voy a marcharme hoy por la tarde .")
        assert len(word_graph.arcs) == 2
        assert len(word_graph.final_states) == 1


def test_get_word_graph_fast_align() -> None:
    with _create_fast_align_model() as smt_model:
        word_graph = smt_model.get_word_graph("voy a marcharme hoy por la tarde .")
        assert len(word_graph.arcs) == 2
        assert len(word_graph.final_states) == 1

Reviewed 1 of 1 files at r1, all commit messages.
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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (96a6786) 86.74% compared to head (c67806b) 86.84%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #77      +/-   ##
==========================================
+ Coverage   86.74%   86.84%   +0.09%     
==========================================
  Files         226      226              
  Lines       13596    13606      +10     
==========================================
+ Hits        11794    11816      +22     
+ Misses       1802     1790      -12     

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[johnml1135]

Reviewed 1 of 1 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @isaac091)