refactor: collect dataframe as stream in __repr__

[konjac]

Which issue does this PR close?

Closes #1014.

Rationale for this change

The root cause for #1014 is _repr_ appends a limit clause and causes EXPLAIN not the top plan.

What changes are included in this PR?

This change refactors _repr_ to collect the dataframe as a stream.

Before the change,

>>> from datafusion import SessionContext
>>> ctx = SessionContext()
>>> ctx.sql("explain select 1")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/kuhuan/.local/lib/python3.10/site-packages/datafusion/dataframe.py", line 71, in __repr__
    return self.df.__repr__()
Exception: Internal error: Unsupported logical plan: Explain must be root of the plan.
This was likely caused by a bug in DataFusion's code and we would welcome that you file an bug report in our issue tracker

After the change

>>> from datafusion import SessionContext
>>> ctx = SessionContext()
>>> ctx.sql("explain select 1")
DataFrame()
+---------------+--------------------------------------+
| plan_type     | plan                                 |
+---------------+--------------------------------------+
| logical_plan  | Projection: Int64(1)                 |
|               |   EmptyRelation                      |
| physical_plan | ProjectionExec: expr=[1 as Int64(1)] |
|               |   PlaceholderRowExec                 |
|               |                                      |
+---------------+--------------------------------------+

Are there any user-facing changes?

No. Bugfix only.

[konjac]

Hi @timsaucer , could you help to trigger CI and also review the proposed change? Thank you!

[timsaucer]

This looks good. It took me a while to parse through the logic of the get_batches. I think it's worth adding some documentation within the file to explain why we are doing this so that future maintainers can understand the necessity of doing it this way.

Thank you for the PR!

[konjac]

This looks good. It took me a while to parse through the logic of the get_batches. I think it's worth adding some documentation within the file to explain why we are doing this so that future maintainers can understand the necessity of doing it this way.

Thank you for the PR!

Thank you for the reviewing! A new iteration is pushed to add code comments.