Promote in convolution (#5727)

Currently this model: ``` m = timm.create_model("efficientformerv2_s0", pretrained=True, scriptable=True).eval()` ``` will fail to convert to stablehlo because we use mixed shape in Conv. this is allowed in HLO but not in mhlo. Workaroudn by manually promoting

Author: qihqi

torch_xla/csrc/convolution.cpp

@@ -348,7 +348,8 @@ xla::XlaOp BuildConvolutionOverrideableBias(
  xla::XlaOp bias_broadcast =
  xla::Transpose(xla::Broadcast(bias, broadcast_sizes),
  BiasTransposePermutation(broadcast_sizes.size() + 1));
- return conv + bias_broadcast;
+ auto promoted = XlaHelpers::Promote(conv, bias_broadcast);
+ return promoted.first + promoted.second;
 }
 
 ConvGrads BuildConvolutionBackwardOverrideable(