Fix cat shape compliance

Summary:
Special handling for empty tensors.

Still some compliance issues. However, that's some undefined behavior from aten. The impl itself is consistent to the definition (https://pytorch.org/docs/stable/generated/torch.cat.html).
```
  ⚠ fbsource//xplat/executorch/kernels/test:portable_op_cat_test - OpCatOutTest.ShapeTest_I0_000I_2_0
  ⚠ fbsource//xplat/executorch/kernels/test:portable_op_cat_test - OpCatOutTest.ShapeTest_I0_0I_1_0
  ⚠ fbsource//xplat/executorch/kernels/test:portable_op_cat_test - OpCatOutTest.ShapeTest_I0I_neg2_0
  ⚠ fbsource//xplat/executorch/kernels/test:portable_op_cat_test - OpCatOutTest.ShapeTest_I0_00I_1_0
  ⚠ fbsource//xplat/executorch/kernels/test:portable_op_cat_test - OpCatOutTest.ShapeTest_I0_00I_neg2_0
  ⚠ fbsource//xplat/executorch/kernels/test:portable_op_cat_test - OpCatOutTest.ShapeTest_I0_000I_1_0
  ⚠ fbsource//xplat/executorch/kernels/test:portable_op_cat_test - OpCatOutTest.ShapeTest_I0_0I_neg2_0
  ⚠ fbsource//xplat/executorch/kernels/test:portable_op_cat_test - OpCatOutTest.ShapeTest_I0_000I_neg3_0
  ⚠ fbsource//xplat/executorch/kernels/test:portable_op_cat_test - OpCatOutTest.ShapeTest_I0_0I_neg3_0
  ⚠ fbsource//xplat/executorch/kernels/test:portable_op_cat_test - OpCatOutTest.ShapeTest_I0_0I_3_0
```

Reviewed By: SS-JIA

Differential Revision: D48420994

fbshipit-source-id: 7f564c4db50dd095617a6209243a1815df9c39d2