Support DataFrame indexing with mars Tensor

Currently the following snippets causes error, which is quite surprising for users coming from Pandas background:
```python
import mars.dataframe as md
import mars.tensor as mt
df = md.DataFrame(mt.random.randn(10, 4))
df.loc[df.index].execute()
```
It will be nice to support indexing with Mars Tensor as well.

Thank you !

Start by reproducing the DataFrame and Mars Tensor example in the issue, then trace DataFrame .loc indexing and the tensor-backed index path. Add coverage for df.loc[df.index].execute() and confirm the example completes without the reported error.

Written by the indexing model from the issue text.