!8760 Add broadcast for a and x support for IGamma

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

mindspore/nn/layer/math.py

@@ -646,10 +646,13 @@ class IGamma(Cell):
         x_dtype = self.dtype(x)
         _check_input_dtype("input_a", a_dtype, [mstype.float16, mstype.float32], self.cls_name)
         _check_input_dtype("input_x", x_dtype, a_dtype, self.cls_name)
-        x_is_zero = self.equal(x, 0)
         domain_error = self.logicalor(self.less(x, 0), self.less(a, 0))
         use_igammac = self.logicaland(self.greater(x, 1), self.greater(x, a))
         ax = a * self.log(x) - x - self.lgamma(a)
+        boradcastto = P.BroadcastTo(self.shape(ax))
+        a = boradcastto(a)
+        x = boradcastto(x)
+        x_is_zero = self.equal(x, 0)
         if a_dtype == mstype.float16:
             log_maxfloat = self.log_maxfloat16
         else: