[BUG] Fix actor pool project splitting when column is not renamed

[kevinzwang]

Previously, this would fail:

import os

os.environ["DAFT_ENABLE_ACTOR_POOL_PROJECTIONS"] = "1"

import daft
from daft import udf

@udf(
    return_dtype=daft.DataType.int64(),
    batch_size=1
)
class MyUDF:
    def __init__(self):
        # import time
        # time.sleep(10)
        pass

    def __call__(self, _):
        
        # import time
        # time.sleep(10)

        import os

        pid = os.getpid()
        return [pid]

MyUDF = MyUDF.with_concurrency(4)

df = daft.from_pydict({"a": list(range(10))})
df = df.into_partitions(4)
df = df.select(MyUDF(df["a"]))
df = df.select(MyUDF(df["a"]))
df.show()

This is because when we split the project into multiple actor pool projects, we create new names for intermediate columns and lose the information about the original name. This PR fixes that by adding an alias to the end of the actor pool projects.

[codspeed-hq]

CodSpeed Performance Report

Merging #2998 will not alter performance

_{Comparing kevin/split-actor-pool-alias (e558ac7) with main (cd59c73)}

Summary

✅ 17 untouched benchmarks

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.12%. Comparing base (a62d276) to head (e558ac7).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2998      +/-   ##
==========================================
+ Coverage   77.80%   78.12%   +0.31%     
==========================================
  Files         602      602              
  Lines       71892    71461     -431     
==========================================
- Hits        55938    55830     -108     
+ Misses      15954    15631     -323     

Files with missing lines	Coverage Δ
...al_optimization/rules/split_actor_pool_projects.rs	95.15% <100.00%> (+0.39%)	⬆️

... and 18 files with indirect coverage changes

[jaychia]

Awesome

[jaychia]

Excellent :)