Apply C++ lints (#830)

Author: scotts

src/torchcodec/_core/Cache.h

@@ -4,6 +4,8 @@
 // This source code is licensed under the BSD-style license found in the
 // LICENSE file in the root directory of this source tree.
 
+#pragma once
+
 #include <torch/types.h>
 #include <memory>
 #include <mutex>
@@ -27,7 +29,7 @@ class Cache {
  public:
   using element_type = std::unique_ptr<T, D>;
 
-  Cache(int capacity) : capacity_(capacity) {}
+  explicit Cache(int capacity) : capacity_(capacity) {}
 
   // Adds an object to the cache if the cache has capacity. Returns true
   // if object was added and false otherwise.
@@ -56,8 +58,9 @@ bool Cache<T, D>::addIfCacheHasCapacity(element_type&& obj) {
 template <typename T, typename D>
 typename Cache<T, D>::element_type Cache<T, D>::get() {
   std::scoped_lock lock(mutex_);
-  if (cache_.empty())
+  if (cache_.empty()) {
     return nullptr;
+  }
 
   element_type obj = std::move(cache_.back());
   cache_.pop_back();
@@ -92,7 +95,15 @@ class PerGpuCache {
   std::vector<std::unique_ptr<Cache<T, D>>> cache_;
 };
 
-torch::DeviceIndex getNonNegativeDeviceIndex(const torch::Device& device) {
+// Note: this function is inline for convenience, not performance. Because the
+// rest of this file is template functions, they must all be defined in this
+// header. This function is not a template function, and should, in principle,
+// be defined in a .cpp file to preserve the One Definition Rule. That's
+// annoying for such a small amount of code, so we just inline it. If this file
+// grows, and there are more such functions, we should break them out into a
+// .cpp file.
+inline torch::DeviceIndex getNonNegativeDeviceIndex(
+    const torch::Device& device) {
   torch::DeviceIndex deviceIndex = device.index();
   // For single GPU machines libtorch returns -1 for the device index. So for
   // that case we set the device index to 0. That's used in per-gpu cache