Slice with Length specified by Expr in Lazy Context cannot Determine Length Correctly

[abstractqqq]

Checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

df = pl.DataFrame({
    "a": [1,2,1,2,1,2],
    "b": [None, None, 1,1,1,1]
})
# Runs perfectly
df.select(
    pl.col("a").slice(offset = 0, length=pl.count() - 2).alias("1"),
    pl.col("a").slice(offset = 1, length=pl.count() - 2).alias("2")
)
# Will panic because it cannot determine the length correctly
df.lazy().select(
    pl.col("a").slice(offset = 0, length=pl.count() - 2).alias("1"),
    pl.col("a").slice(offset = 1, length=pl.count() - 2).alias("2")
).collect()

Log output

No response

Issue description

pl.col("").slice(), with length given as an expression cannot determine the correct selected column length.

PanicException: The column lengths in the DataFrame are not equal.

This is likely a problem in the order of execution.

Expected behavior

Eager and lazy should return the same result

Installed versions

--------Version info---------
Polars:              0.19.7
Index type:          UInt32
Platform:            Windows-10-10.0.19045-SP0
Python:              3.11.3 | packaged by Anaconda, Inc. | (main, Apr 19 2023, 23:46:34) [MSC v.1916 64 bit (AMD64)]

[orlp]

I think a more informative example of what goes wrong is this:

>>> df = pl.DataFrame({
...     "a": [1,2,1,2,1,2],
...     "b": [None, None, 1,1,1,1]
... })
>>> df.lazy().select(
...     pl.col("a").slice(offset = 1, length=pl.count() - 1).alias("1"),
...     pl.col("a").slice(offset = 1, length=pl.count() - 1).alias("2")
... ).collect()
shape: (0, 2)
┌─────┬─────┐
│ 1   ┆ 2   │
│ --- ┆ --- │
│ u32 ┆ u32 │
╞═════╪═════╡
└─────┴─────┘