[DOC] Improve Code Coverage in Transformers4Rec to X%

[bschifferer]

Reviewed API documentation (generated by docstrings in code) for Transformers4Rec/transformers4rec codebase. Details are available here: https://docs.google.com/document/d/1GzM1q9Pc6amecRspbWKG0PSXbU-fCy13hSOC6UMnWWU/edit

Describe the documentation you'd like

Inline Code Coverage of Transformers4Rec is around 40%. We should aim for X%.

Transformers4Rec:
========================== Coverage for /workspace/01_MerlinDev/62_DocStrings/Transformers4Rec/ ===========================
--------------------------------------------------------- Summary ---------------------------------------------------------

Name	Total	Miss	Cover	Cover%
merlin_standard_lib/proto/schema_bp.py	47	8	39	83%
merlin_standard_lib/schema/schema.py	30	29	1	3%
merlin_standard_lib/utils/embedding_utils.py	2	2	0	0%
transformers4rec/config/schema.py	6	6	0	0%
transformers4rec/config/transformer.py	22	22	0	0%
transformers4rec/data/dataset.py	4	4	0	0%
transformers4rec/torch/experimental.py	4	3	1	25%
transformers4rec/torch/losses.py	2	1	1	50%
transformers4rec/torch/masking.py	19	8	11	58%
transformers4rec/torch/ranking_metric.py	9	8	1	11%
transformers4rec/torch/trainer.py	19	6	13	68%
transformers4rec/torch/block/base.py	19	19	0	0%
transformers4rec/torch/block/mlp.py	4	4	0	0%
transformers4rec/torch/block/transformer.py	8	5	3	38%
transformers4rec/torch/features/base.py	1	1	0	0%
transformers4rec/torch/features/continuous.py	4	3	1	25%
transformers4rec/torch/features/embedding.py	17	11	6	35%
transformers4rec/torch/features/sequence.py	9	6	3	33%
transformers4rec/torch/features/tabular.py	4	1	3	75%
transformers4rec/torch/model/base.py	31	20	11	35%
transformers4rec/torch/model/prediction_task.py	14	11	3	21%
transformers4rec/torch/tabular/aggregation.py	13	9	4	31%
transformers4rec/torch/tabular/base.py	29	15	14	48%
transformers4rec/torch/tabular/transformations.py	12	9	3	25%
transformers4rec/torch/utils/data_utils.py	14	8	6	43%
transformers4rec/torch/utils/examples_utils.py	4	1	3	75%
transformers4rec/torch/utils/schema_utils.py	1	1	0	0%
transformers4rec/torch/utils/torch_utils.py	29	16	13	45%
transformers4rec/utils/data_utils.py	4	2	2	50%
transformers4rec/utils/dependencies.py	3	3	0	0%
-------------------------------------------------------------	--------------	-------------	--------------	---------------
TOTAL	413	242	171	41.4%
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