Add benchmark for calculate_qparams (pytorch#42138)

Summary:
Adds a benchmark for `HistogramObserver.calculate_qparams` to the quantized op benchmarks. The next diff in this stack adds a ~15x speedup for this benchmark.

Pull Request resolved: pytorch#42138

Test Plan:
While in the folder `benchmarks/operator_benchmark`, the benchmark can be run using `python -m benchmark_all_quantized_test --operators HistogramObserverCalculateQparams`.

Benchmark results before speedup:
```
# ----------------------------------------
# PyTorch/Caffe2 Operator Micro-benchmarks
# ----------------------------------------
# Tag : short

# Benchmarking PyTorch: HistogramObserverCalculateQparams
# Mode: Eager
# Name: HistogramObserverCalculateQparams_C3_M512_N512_dtypetorch.quint8_cpu_qschemetorch.per_tensor_affine
# Input: C: 3, M: 512, N: 512, dtype: torch.quint8, device: cpu, qscheme: torch.per_tensor_affine
Forward Execution Time (us) : 185818.566

# Benchmarking PyTorch: HistogramObserverCalculateQparams
# Mode: Eager
# Name: HistogramObserverCalculateQparams_C3_M512_N512_dtypetorch.quint8_cpu_qschemetorch.per_tensor_symmetric
# Input: C: 3, M: 512, N: 512, dtype: torch.quint8, device: cpu, qscheme: torch.per_tensor_symmetric
Forward Execution Time (us) : 165325.916
```

Benchmark results after speedup:
```
# ----------------------------------------
# PyTorch/Caffe2 Operator Micro-benchmarks
# ----------------------------------------
# Tag : short

# Benchmarking PyTorch: HistogramObserverCalculateQparams
# Mode: Eager
# Name: HistogramObserverCalculateQparams_C3_M512_N512_dtypetorch.quint8_cpu_qschemetorch.per_tensor_affine
# Input: C: 3, M: 512, N: 512, dtype: torch.quint8, device: cpu, qscheme: torch.per_tensor_affine
Forward Execution Time (us) : 12242.241

# Benchmarking PyTorch: HistogramObserverCalculateQparams
# Mode: Eager
# Name: HistogramObserverCalculateQparams_C3_M512_N512_dtypetorch.quint8_cpu_qschemetorch.per_tensor_symmetric
# Input: C: 3, M: 512, N: 512, dtype: torch.quint8, device: cpu, qscheme: torch.per_tensor_symmetric
Forward Execution Time (us) : 12655.354
```

Reviewed By: supriyar

Differential Revision: D22779291

Pulled By: durumu

fbshipit-source-id: 1fe17d20eda5dd99e0e2590480142034c3574d4e

benchmarks/operator_benchmark/pt/qobserver_test.py

@@ -70,6 +70,13 @@
     ]
 )
 
+qobserver_calculate_qparams_list = op_bench.op_list(
+    attr_names=['op_name', 'op_func'],
+    attrs=[
+        ['HistogramObserverCalculateQparams', obs.HistogramObserver],
+    ]
+)
+
 
 class QObserverBenchmark(op_bench.TorchBenchmarkBase):
     def init(self, C, M, N, dtype, qscheme, op_func, device):
@@ -79,6 +86,15 @@ def init(self, C, M, N, dtype, qscheme, op_func, device):
     def forward(self):
         return self.op_func(self.f_input)
 
+class QObserverBenchmarkCalculateQparams(op_bench.TorchBenchmarkBase):
+    def init(self, C, M, N, dtype, qscheme, op_func, device):
+        self.f_input = torch.rand(C, M, N, device=device)
+        self.q_observer = op_func(dtype=dtype, qscheme=qscheme).to(device)
+        self.q_observer(self.f_input)
+
+    def forward(self):
+        return self.q_observer.calculate_qparams()
+
 
 op_bench.generate_pt_tests_from_op_list(
     qobserver_per_tensor_list,
@@ -90,6 +106,11 @@ def forward(self):
     qobserver_per_channel_configs_short + qobserver_per_channel_configs_long,
     QObserverBenchmark)
 
+op_bench.generate_pt_tests_from_op_list(
+    qobserver_calculate_qparams_list,
+    qobserver_per_tensor_configs_short + qobserver_per_tensor_configs_long,
+    QObserverBenchmarkCalculateQparams)
+
 
 if __name__ == "__main__":
     op_bench.benchmark_runner.main()