lowering pass: fully remove SymInt

[zewenli98]

Description

Added a lowering pass to replace SymInt with aten.sym_size. Users now can explicitly use dynamic dims in the models. e.g.:

import logging

import torch
import torch.nn as nn
import torch_tensorrt

logging.basicConfig(level=logging.DEBUG)

torch.manual_seed(0)


class ExpandReshapeModel(nn.Module):
    def __init__(self, embed_dim: int):
        super().__init__()
        self.cls_token = nn.Parameter(torch.randn(1, 1, embed_dim))
        self.embed_dim = embed_dim
        self.qkv_proj = nn.Linear(self.embed_dim, self.embed_dim * 3)

    def forward(self, x: torch.Tensor):
        batch_size = x.shape[0]  # dynamic dim
        cls_token = self.cls_token.expand(batch_size, -1, -1)
        x = torch.cat([cls_token, x], dim=1)
        x = self.qkv_proj(x)
        reshaped_qkv = x.reshape(batch_size, x.size(1), 3, 12, -1)
        return reshaped_qkv


model = ExpandReshapeModel(embed_dim=768).cuda().eval()
x = torch.randn(4, 196, 768).cuda()
torch._dynamo.mark_dynamic(x, index=0, min=2, max=32)
trt_module = torch.compile(model, backend="tensorrt")
out = trt_module(x)
print(out)

Fixes #3981

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the style guidelines of this project (You can use the linters)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas and hacks
• I have made corresponding changes to the documentation
• I have added tests to verify my fix or my feature
• New and existing unit tests pass locally with my changes
• I have added the relevant labels to my PR in so that relevant reviewers are notified