Sample custom guards [Do not land, for answering question]

[voznesenskym]

Context:
https://dev-discuss.pytorch.org/t/can-backend-compilers-modify-guards-for-torch-dynamo-frame/1074

Invoked with:

def my_backend(gm, args):
    print("Compile!")
    TracingContext.get().guards_context.custom_guards.append(ArbitraryStringGuard("x.tag == 'foo'"))
    return gm
    
x = torch.randn([2, 2])
y = torch.randn([2, 2])

def simple_fn(x, y):
    return x * y    

torch._dynamo.optimize(my_backend)(simple_fn)(x, y)
x.tag = 'bar'
torch._dynamo.optimize(my_backend)(simple_fn)(x, y)
x.tag = 'foo'
torch._dynamo.optimize(my_backend)(simple_fn)(x, y)
torch._dynamo.optimize(my_backend)(simple_fn)(x, y)

Run with TORCHDYNAMO_PRINT_GUARDS=1

To see:

GUARDS ___guarded_code.valid and ___check_tensors(x, y) and x.tag == 'foo' and x.size()[1] == x.size()[0] and x.stride()[0] == x.size()[0] and x.stride()[1] == 1 and x.storage_offset() == 0 and y.size()[0] == x.size()[0] and y.size()[1] == x.size()[0] and y.stride()[0] == x.size()[0] and y.stride()[1] == 1 and y.storage_offset() == 0 and Eq(x.size()[0], 2) and 2 <= x.size()[0]

cc @soumith @yanboliang @penguinwu @anijain2305 @EikanWang @jgong5 @Guobing-Chen @XiaobingSuper @zhuhaozhe @blzheng @Xia-Weiwen @wenzhe-nrv @jiayisunx @desertfire

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[ezyang]

do you want review