Create metrics for a single sequence of data

[npatki]

Problem Description

Existing sequential metrics such as LSTM Detection are designed to work only when there are entity_columns present -- i.e. when there are multiple sequences present in the data.

Sometimes, users may have datasets where there are no entity columns. This means that the data represents only a single sequence.

See issues:

• SDMetrics: Running a detection metric on time series data with no entity_columns fails #77
• SDV: #487

Expected behavior

Consider adding new metric(s) specifically for the case outlined above.