[Relax][PyTorch] Cleanup Neural Network op converters (#17369)

* cleanup `_adaptive_avg_pool2d()`

* cleanup `addmm()`

* cleanup `_avg_pool2d()`

* cleanup `_baddbmm()`

* cleanup `_conv1d_transpose()`

* cleanup `_conv2d_transpose()`

* cleanup `_conv1d()`

* cleanup `_conv2d()`

* cleanup `_conv3d()`

* cleanup `_einsum()`

* cleanup `_embedding()`

* cleanup `_group_norm()`

* cleanup `_layer_norm()`

* cleanup `_linear()`

* cleanup `_max_pool2d()`

* cleanup `_scaled_dot_product_attention()`

* cleanup `_unbind()`

* remove `_matmul_impl()` since we don't use it anymore