!8482 Add IGamma operator

From: @peixu_ren
Reviewed-by: @zh_qh
Signed-off-by:

tests/ut/python/ops/test_nn_ops.py

@@ -593,6 +593,11 @@ test_cases = [
         'block': nn.LGamma(),
         'desc_inputs': [Tensor(np.array([3, 4, 5, 6]).astype(np.float32))],
         'skip': ['backward']}),
+    ('IGamma', {
+        'block': nn.IGamma(),
+        'desc_inputs': [Tensor(np.array([3, 4, 5, 6]).astype(np.float32)),
+                        Tensor(np.array([3, 4, 5, 6]).astype(np.float32))],
+        'skip': ['backward']}),
     ('FlattenNet', {
         'block': FlattenNet(),
         'desc_inputs': [Tensor(np.ones([1, 2, 3, 4], np.float32))],