log_metric: raise helpful error if a nested metric is logged and parent key exists as a metric

[dberenbaum]

This could happen without treating step as a metric. For example, even without step, you could get the same error for epoch here if you include epoch-level metrics.

We should fail in log_metric if a nested metric (foo/bar) is logged and any of the parent keys already exists in the summary.

Originally posted by @daavoo in #490 (comment)

[dberenbaum]

See #490 (comment) for more context.

[dberenbaum]

Summary: If you have already logged a metric like {"step": 0} and try to log a nested metric like {"step": {"loss": 0.5}}, you will get an error like:

  File "/Users/dave/Code/dvclive/src/dvclive/utils.py", line 35, in nested_update
    d[k] = nested_update(d.get(k, {}), v)
AttributeError: 'int' object has no attribute 'get'