Backend paddle: allow enable prim to accelerate running

Author: lijialin03

deepxde/backend/paddle/__init__.py

@@ -1 +1,16 @@
-from .tensor import *  # pylint: disable=redefined-builtin
+from .tensor import *  # pylint: disable=redefined-builtin
+import os
+
+# enable prim if specified
+enable_prim_value = os.getenv("PRIM")
+enable_prim = enable_prim_value.lower() in ['1', 'true', 'yes', 'on'] if enable_prim_value else False
+if enable_prim:
+    # Mostly for compiler running with dy2st.
+    from paddle.framework import core
+
+    core.set_prim_eager_enabled(True)
+    # The following protected member access is required.
+    # There is no alternative public API available now.
+    # pylint: disable=protected-access
+    core._set_prim_all_enabled(True)
+    print("Prim mode is enabled.")