Add a __str__ function for Point (meta-pytorch#950)

Summary:
Pull Request resolved: meta-pytorch#950

str(current_rank()) in an actor doesn't print anything meaningful

I added a `__repr__` function to address that

It prints something like this:

```
rank=0/2 coords={hosts=0/1,gpus=0/2}
```

Reviewed By: dulinriley

Differential Revision: D80649558

fbshipit-source-id: cd8d126b98d6ae0006872c882d022205410f998d

Author: ahmadsharif1

monarch_hyperactor/src/shape.rs

@@ -210,6 +210,36 @@ impl PyPoint {
         }
     }
 
+    fn __repr__(&self, py: Python) -> PyResult<String> {
+        let shape = self.shape.bind(py).get();
+        let inner_shape = &shape.inner;
+        let slice = inner_shape.slice();
+
+        let total_size = slice.len();
+        let current_rank = self.rank;
+
+        let coords = slice
+            .coordinates(current_rank)
+            .map_err(|e| PyErr::new::<PyValueError, _>(e.to_string()))?;
+
+        let labels = inner_shape.labels();
+        let sizes = slice.sizes();
+
+        let coords_parts: Vec<String> = labels
+            .iter()
+            .zip(coords.iter())
+            .zip(sizes.iter())
+            .map(|((label, &coord), &size)| format!("{}={}/{}", label, coord, size))
+            .collect();
+
+        let coords_str = coords_parts.join(",");
+
+        Ok(format!(
+            "rank={}/{} coords={{{}}}",
+            current_rank, total_size, coords_str
+        ))
+    }
+
     fn __len__(&self, py: Python) -> usize {
         self.shape.bind(py).get().__len__()
     }

python/tests/test_python_actors.py

@@ -163,6 +163,10 @@ class RunIt(Actor):
     async def run(self, fn):
         return fn()
 
+    @endpoint
+    async def return_current_rank_str(self):
+        return str(current_rank())
+
 
 @pytest.mark.timeout(60)
 async def test_rank_size():
@@ -175,6 +179,17 @@ async def test_rank_size():
     assert 4 == await acc.accumulate(lambda: current_size()["gpus"])
 
 
+@pytest.mark.timeout(60)
+async def test_rank_string():
+    proc = await local_proc_mesh(gpus=2)
+    r = await proc.spawn("runit", RunIt)
+    vm = r.return_current_rank_str.call().get()
+    r0 = vm.flatten("r").slice(r=0).item()
+    r1 = vm.flatten("r").slice(r=1).item()
+    assert r0 == "rank=0/2 coords={hosts=0/1,gpus=0/2}"
+    assert r1 == "rank=1/2 coords={hosts=0/1,gpus=1/2}"
+
+
 class SyncActor(Actor):
     @endpoint
     def sync_endpoint(self, a_counter: Counter):