HeteroGraphConv documentation sample code crash

## 🐛 Bug

## To Reproduce

Steps to reproduce the behavior:
Following the code snippet example in HeteroGraphConv documentation with an actual implementation results in a crash
[HeteroGraphConv](https://docs.dgl.ai/en/latest/generated/dgl.nn.pytorch.HeteroGraphConv.html)

```python
import dgl
import dgl.graphbolt as gb
import dgl.nn as dglnn

import torch as th
import numpy as np

n_users = 50
n_games = 10
n_stores = 5

follows_src = np.random.randint(0, n_users, 100)
follows_dst = np.random.randint(0, n_users, 100)

plays_src = np.random.randint(0, n_users, 50)
plays_dst = np.random.randint(0, n_games, 50)

sells_src = np.random.randint(0, n_stores, 20)
sells_dst = np.random.randint(0, n_games, 20)

g = dgl.heterograph(
{
("user", "follows", "user"): (follows_src, follows_dst),
("user", "plays", "game"): (plays_src, plays_dst),
("store", "sells", "game"): (sells_src, sells_dst),
}
)

input_dim = 16
out_dim = 8

hetero_conv = dglnn.HeteroGraphConv(
{
"follows": dglnn.GraphConv(input_dim, out_dim),
"plays": dglnn.GraphConv(input_dim, out_dim),
"sells": dglnn.GraphConv(input_dim, out_dim),
},
aggregate="sum",
)

h1 = {"user": th.randn((g.num_nodes("user"), input_dim))}

h2 = hetero_conv(g, h1)
print(h2.keys())
```

```shell
python heterographconv_example.py
Traceback (most recent call last):
File "/Users/yuri/remix/projects/easy-platform/platform-python/pinsage-recommender/heterographconv_example.py", line 44, in
h2 = hetero_conv(g, h1)
^^^^^^^^^^^^^^^^^^
File "/Users/yuri/remix/projects/easy-platform/platform-python/pinsage-recommender/.venv/lib/python3.11/site-packages/torch/nn/modules/module.py", line 1532, in _wrapped_call_impl
return self._call_impl(*args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/yuri/remix/projects/easy-platform/platform-python/pinsage-recommender/.venv/lib/python3.11/site-packages/torch/nn/modules/module.py", line 1541, in _call_impl
return forward_call(*args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/yuri/remix/projects/easy-platform/platform-python/pinsage-recommender/.venv/lib/python3.11/site-packages/dgl-2.3.0-py3.11-macosx-11.1-arm64.egg/dgl/nn/pytorch/hetero.py", line 212, in forward
(inputs[stype], inputs[dtype]),
~~~~~~^^^^^^^
KeyError: 'game'
```
## Expected behavior
```shell
dict_keys(['user', 'game'])
```

## Environment

- DGL Version: 2.3.0
- OS: macOS Sonoma 14.5
- How you installed DGL: source (HEAD detached at v2.3.0)
- Build command you used:
```shell
cmake -DBUILD_CPP_TEST=OFF -DUSE_OPENMP=OFF -DUSE_LIBXSMM=OFF ..
make -j4
```
- Python version: Python 3.11.5

## Additional context

When 'game' features are added to **h1** the example works
Not sure, if this is an implementation bug or a documentation issue.
It seems reasonable to expect the example code to work as shown in documentation in case 'game' nodes don't have any intrinsic features
Also, when a block is passed into the forward function instead of a graph, the failure is silent and the call returns an empty dictionary