Prevent stack overflow on deletion of deep graph (#6873)

* Prevent stack overflow on deletion of deep graph

Fixes #5534.

Sometimes one can end up with a very big computation graph of Functions
and Edges. Each std::shared_ptr<Function> contains a list of Edge, and
each Edge contains a std::shared_ptr<Function>. Deleting a
std::shared_ptr<Function> can trigger the recursive deletion of other
std::shared_ptr<Function>'s: this can stack overflow if the graph
is deep enough. Here is an example of such a graph:

    shared_ptr<Function> -> Edge -> shared_ptr<Function> -> Edge -> ... -> shared_ptr<Function>

The solution here is to use a custom deleter with each
std::shared_ptr<Function>. The custom deleter keeps track of how many
nested deleters it is in. When this number exceeds the maximum allowed
depth, the Function* to be deleted are accumulated in a per-thread
delete queue and handled by one of the deleters.

Example code that could trigger the overflow (set ``depth`` to something >
100000) is below. I also benchmarked the below code before/after the
changes to see if there are any significant performance differences.

```
import torch
def scope():
    depth = 80000
    x = torch.randn(9, requires_grad=True)
    y = x.clone()

    # build deeply nested computation graph
    for i in range(depth):
        y = y + y * 0.000001

%timeit -n 100 scope()

376 ms ± 3.94 ms per loop (mean ± std. dev. of 7 runs, 100 loops each)

Without changes:
352 ms ± 6.58 ms per loop (mean ± std. dev. of 7 runs, 100 loops each)
```

With the change, the above code is 6.8% slower.

UPDATE: I did some more benchmarking. It looks like it takes 25% more time to free the computation graph in the case of the straight chain graph: https://gist.github.com/zou3519/93cf84d96ae431356ae7f7c1923ef51a

* WIP

* Add custom deleter to PyFunctions created by THPFunction

* Address some comments; pick new value

* Address some more comments

* Add more complicated test; special case the windows depth constant

1 files changed, 1 insertions, 1 deletions

diff --git a/tools/autograd/gen_variable_type.py b/tools/autograd/gen_variable_type.py
index c672c447ce..a47c46d422 100644
--- a/tools/autograd/gen_variable_type.py
+++ b/tools/autograd/gen_variable_type.py
@@ -93,7 +93,7 @@ if (compute_requires_grad( ${args_with_derivatives} )) {
 """)
 
 ASSIGN_GRAD_FN = CodeTemplate("""\
-grad_fn = std::make_shared<${op}>(${op_ctor});
+grad_fn = std::shared_ptr<${op}>(new ${op}(${op_ctor}), deleteFunction);
 grad_fn->set_next_edges(collect_next_edges( ${args_with_derivatives} ));
 """)