Update swiglu and geglu forward: zeros_like -> empty_like (#217)

## Summary
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this proposed code change. --->
This PR improves the performance of swiglu and geglu forward by
replacing `zeros_like` with `empty_like`. The difference is that
`empty_like` doesn't require a separate kernel launch.

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## Details
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should be aware of?
--->

## Testing Done
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tested. --->
Testing is covered by existing `test_geglu.py` and `test_swiglu.py`.

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---------

Co-authored-by: Byron Hsu <byronhsu1230@gmail.com>
Co-authored-by: Shao Tang <tangshao28@gmail.com>

src/liger_kernel/ops/geglu.py

@@ -98,7 +98,7 @@ def geglu_forward(a, b):
     n_cols = ori_shape[-1]
     a = a.view(-1, n_cols)
     b = b.view(-1, n_cols)
-    c = torch.zeros_like(a)
+    c = torch.empty_like(a)
     n_rows = a.shape[0]
 
     BLOCK_SIZE, num_warps = calculate_settings(n_cols)

src/liger_kernel/ops/swiglu.py

@@ -66,7 +66,7 @@ def swiglu_forward(a, b):
     n_cols = ori_shape[-1]
     a = a.view(-1, n_cols)
     b = b.view(-1, n_cols)
-    c = torch.zeros_like(a)
+    c = torch.empty_like(a)
     n_rows = a.shape[0]
 
     BLOCK_SIZE, num_warps = calculate_settings(n_cols)