Fix jax enable x64 (#5960)

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------------------------------------------------------------------------------------------------------------

**Context:**
@dwierichs reported the following 
```
import jax
import pennylane as qml

x = jax.numpy.array(0.4, dtype=jax.numpy.float32)

@qml.qnode(qml.device("default.qubit", shots=10))
def node(x):
    qml.RX(x, 0)
    qml.measure(0)
    return qml.expval(qml.Z(0))

print(jax.config.jax_enable_x64)
>>> False # Expected because it was never activated
out = node(x)
print(jax.config.jax_enable_x64)
>>> True # Not expected.
```

**Description of the Change:**
`qml.compiler.active` first checks whether Catalyst is imported at all
to avoid changing `jax_enable_x64` on module initialization.

**Benefits:**

**Possible Drawbacks:**

**Related GitHub Issues:**
[sc-67969]

---------

Co-authored-by: Astral Cai <[email protected]>

doc/releases/changelog-0.37.0.md

@@ -738,6 +738,9 @@ Stay tuned for an in-depth demonstration on using this feature with real-world e
 
 <h3>Bug fixes 🐛</h3>
 
+* `qml.compiler.active` first checks whether Catalyst is imported at all to avoid changing `jax_enable_x64` on module initialization.
+  [(#5960)](https://github.com/PennyLaneAI/pennylane/pull/5960)
+
 * The `__invert__` dunder method of the `MeasurementValue` class uses an array-valued function.
   [(#5955)](https://github.com/PennyLaneAI/pennylane/pull/5955)
 

pennylane/compiler/compiler.py

@@ -15,6 +15,7 @@
 
 import dataclasses
 import re
+import sys
 from collections import defaultdict
 from importlib import metadata, reload
 from sys import version_info
@@ -206,6 +207,8 @@ def circuit(phi, theta):
     """
 
     for name, eps in AvailableCompilers.names_entrypoints.items():
+        if name not in sys.modules:
+            continue
         tracer_loader = eps["context"].load()
         if tracer_loader.is_tracing():
             return name
@@ -245,5 +248,4 @@ def circuit(phi, theta):
     >>> qml.qjit(circuit)(np.pi, np.pi / 2)
     -1.0
     """
-
     return active_compiler() is not None

tests/test_compiler.py

@@ -105,6 +105,14 @@ def circuit(phi, theta):
         assert jnp.allclose(circuit(jnp.pi, jnp.pi / 2), 1.0)
         assert jnp.allclose(qml.qjit(circuit)(jnp.pi, jnp.pi / 2), -1.0)
 
+    @pytest.mark.parametrize("jax_enable_x64", [False, True])
+    def test_jax_enable_x64(self, jax_enable_x64):
+        """Test whether `qml.compiler.active` changes `jax_enable_x64`."""
+        jax.config.update("jax_enable_x64", jax_enable_x64)
+        assert jax.config.jax_enable_x64 is jax_enable_x64
+        qml.compiler.active()
+        assert jax.config.jax_enable_x64 is jax_enable_x64
+
     def test_qjit_circuit(self):
         """Test JIT compilation of a circuit with 2-qubit"""
         dev = qml.device("lightning.qubit", wires=2)