Fix arm_backend.py script (pytorch#1120)

Summary:
data["scratch_shape"][0] is a number.

Pull Request resolved: pytorch#1120

Reviewed By: digantdesai

Differential Revision: D50863106

Pulled By: kirklandsign

fbshipit-source-id: 933ed4750a2f615244d5d8af01ff12f0814bb6b2

backends/arm/arm_backend.py

@@ -196,6 +196,8 @@ def vela_compile(tosa_fb):
             # Add a block for scratch, inputs and outputs;  scratch shape is a 1 element
             # array giving us size in bytes so extract this and add a block of 0's.
             # Currently we preallocated this on the host to provide SRAM for computation.
+            if not isinstance(data["scratch_shape"][0], np.int64):
+                raise RuntimeError("Expected scratch to be int64")
             block_length = int(data["scratch_shape"][0])
             bin_blocks["scratch_data"] = b"\x00" * block_length