Fix label access on clustering model

[lhw362950217]

Fixes #3028

[codecov-io]

Codecov Report

Merging #3030 into develop will decrease coverage by 0.24%.
The diff coverage is 57.14%.

@@ Coverage Diff @@ ## develop #3030 +/- ## =========================================== - Coverage 50.44% 50.20% -0.25%  =========================================== Files 248 249 +1 Lines 17823 17897 +74 =========================================== - Hits 8991 8985 -6  - Misses 8142 8218 +76  - Partials 690 694 +4 

Impacted Files	Coverage Δ
go/executor/executor.go	5.31% <ø> (ø)
...thon/runtime/local/tensorflow_submitter/predict.py	26.66% <0.00%> (-0.61%)	⬇️
python/runtime/step/tensorflow/train.py	21.73% <0.00%> (-0.49%)	⬇️
go/ir/ir_generator.go	57.60% <100.00%> (+0.06%)	⬆️
python/runtime/local/create_result_table.py	97.14% <100.00%> (+0.08%)	⬆️
python/runtime/pai/submit_pai_task.py	30.00% <0.00%> (-14.45%)	⬇️
go/executor/fields.go	9.09% <0.00%> (-7.13%)	⬇️
go/cmd/sqlflowserver/testing.go	60.47% <0.00%> (-5.72%)	⬇️
go/ir/derivation.go	62.22% <0.00%> (-3.45%)	⬇️
python/runtime/pai/get_pai_tf_cmd.py	92.68% <0.00%> (-2.19%)	⬇️
... and 14 more

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