Fix buggy unreduced add lowering.
I noticed the following program was producing the wrong gradients:
```python
import jax
import jax.numpy as jnp
jax.config.update('jax_num_cpu_devices', 2)
mesh = jax.make_mesh((2,) , ("i",))
jax.set_mesh(mesh)
x = jax.reshard(jnp.ones((2, 2)), jax.P(None, None, reduced={'i'}))
def f(x):
return jnp.sum(x + x)
y, vjp = jax.vjp(f, x)
g = vjp(1.)[0]
print(g.sharding)
for shard in g.addressable_shards:
print(shard.data)
```
The gradient should be an unreduced array where each device-local shard is [[1,
1], [1, 1]], but we were seeing [[2, 2], [2, 2]] on each shard.
Yash found and fixed the bug.
PiperOrigin-RevId: 878663233
