[Pallas TPU] Better error message for lowering sp.broadcast_to_p

`sp.broadcast_to_p` is a GPU-specific primitive, but it mistakenly appears in TPU lowerings. This PR improves the error message to reflect this.

As an example, currently, users will hit this error when doing:

```
def kernel(x_ref, o_ref):
    m, n = 32, 8
    x = pl.load(x_ref, (jnp.arange(m, dtype=jnp.int32)[:, None], jnp.arange(n, dtype=jnp.int32)[None]))
    o_ref[...] = x
```

PiperOrigin-RevId: 700290975

Author: ayaka14732

jax/_src/pallas/mosaic/lowering.py

@@ -1545,6 +1545,18 @@ def _proxy_reduce(arg, *, axes):
 lowering_rules[lax.reduce_or_p] = _reduce_or_lowering_rule
 
 
+def _broadcast_to_lowering_rule(
+    ctx: LoweringRuleContext, x, shape: Sequence[int]
+):
+  raise RuntimeError(
+      "`broadcast_to` is a Triton-specific primitive. Please consider using"
+      " `jnp.broadcast_to` instead."
+  )
+
+
+lowering_rules[state_primitives.broadcast_to_p] = _broadcast_to_lowering_rule
+
+
 def _broadcast_in_dim_lowering_rule(
     ctx: LoweringRuleContext, val, *, shape, broadcast_dimensions, sharding
 ):