dtype of `y` array in `DiskDataset` depends on whether `X` is accessed beforehand
## 🐛 Bug

```
_, datasets, _ = dc.molnet.load_hiv(
    featurizer=dc.feat.RawFeaturizer(smiles=False),
    splitter=None,   # no splitting
    transformers=[]  # no transformation
)
print(datasets[0].y.dtype)
```
outputs
```
object
```

but
```
_, datasets, _ = dc.molnet.load_hiv(
    featurizer=dc.feat.RawFeaturizer(smiles=False),
    splitter=None,   # no splitting
    transformers=[]  # no transformation
)
print(datasets[0].X.dtype)
print(datasets[0].y.dtype)
```
outputs
```
object
float64
```

In the first case `y` has a `dtype` of `object`, but in the second case it has a `dtype` of `float64`

## To Reproduce

See above.

This is for DeepChem version 2.4.0-rc1.dev, installed via pip.

## Expected behavior

The `dtype` to consistently be `float64`

## Environment

* OS: Ubuntu 18.04. LTS
* Python version: 3.8.6
* DeepChem version: 2.4.0-rc1.dev
* RDKit version (optional): 2020.09.3
diff --git a/deepchem/data/datasets.py b/deepchem/data/datasets.py
index abe66c82b..10af7a99d 100644
--- a/deepchem/data/datasets.py
+++ b/deepchem/data/datasets.py
@@ -2180,8 +2180,7 @@ class DiskDataset(Dataset):
     if self._cached_shards is not None and self._cached_shards[i] is not None:
       return self._cached_shards[i].y
     row = self.metadata_df.iloc[i]
-    return np.array(
-        load_from_disk(os.path.join(self.data_dir, row['y'])), dtype=object)
+    return np.array(load_from_disk(os.path.join(self.data_dir, row['y'])))
 
   def get_shard_w(self, i: int) -> np.ndarray:
     """Retrieves the weights for the i-th shard from disk.
@@ -2200,8 +2199,7 @@ class DiskDataset(Dataset):
     if self._cached_shards is not None and self._cached_shards[i] is not None:
       return self._cached_shards[i].w
     row = self.metadata_df.iloc[i]
-    return np.array(
-        load_from_disk(os.path.join(self.data_dir, row['w'])), dtype=object)
+    return np.array(load_from_disk(os.path.join(self.data_dir, row['w'])))
 
   def add_shard(self,
                 X: np.ndarray,
['deepchem/data/datasets.py']