Move groupby.agg logic into query compiler

[ienkovich]

System information

• OS Platform and Distribution (e.g., Linux Ubuntu 16.04): Linux Ubuntu 19.04
• Modin installed from (source or binary): source
• Modin version: master
• Python version: 3.8.3
• Exact command to reproduce: python test.py

Test case:

import os

os.environ["MODIN_ENGINE"] = "ray"

import modin.pandas as pd

data = {
    "a": [1, 1, 2, 2],
    "b": [11, 21, 12, 11],
}

df = pd.DataFrame(data)
ref = df.groupby("a").agg({"b": "mean"})
print(ref)

In the execution log I see

UserWarning: `DataFrame.groupby_on_multiple_columns` defaulting to pandas implementation.

Previously we could process such aggregates in OmniSci back-end, now it's defaulted to pandas in front-end. That 'breaks' OmniSci backend (processing doesn't happen in OmniSci). Don't know when degradation happened. Probably some code was moved from query compiler to front-end.

[devin-petersohn]

I am going to label this slightly differently. The patch was never upstreamed.