Add user-configurable backtrace limit

A spin off of apache#9872, this adds an env variable `TVM_BACKTRACE_LIMIT` which can be set to an integer to limit the frames printed out on errors. This can make it easier to run interactive TVM scripts with errors since the stack traces are often long (70+ frames).

```bash
export TVM_BACKTRACE_LIMIT=5
python some_code_with_an_error.py
```

cc @tkonolige

src/runtime/logging.cc

@@ -18,6 +18,7 @@
  */
 #include <tvm/runtime/logging.h>
 
+#include <stdexcept>
 #include <string>
 
 #if TVM_LOG_STACK_TRACE
@@ -120,7 +121,22 @@ int BacktraceFullCallback(void* data, uintptr_t pc, const char* filename, int li
 
 std::string Backtrace() {
   BacktraceInfo bt;
-  bt.max_size = 500;
+
+  // Limit backtrace length based on TVM_BACKTRACE_LIMIT env variable
+  auto user_limit_s = getenv("TVM_BACKTRACE_LIMIT");
+  const auto default_limit = 500;
+
+  if (user_limit_s == nullptr) {
+    bt.max_size = default_limit;
+  } else {
+    // Parse out the user-set backtrace limit
+    try {
+      bt.max_size = std::stoi(user_limit_s);
+    } catch (const std::invalid_argument& e) {
+      bt.max_size = default_limit;
+    }
+  }
+
   if (_bt_state == nullptr) {
     return "";
   }