Make ITraceActivity::flowId return int64_t instead of int (#1019)

Summary:
Pull Request resolved: #1019

RoctracerActivity::flowId and CuptiActivity::flowId return correlationId(), which is an int64.

I think we had an instance (https://fburl.com/workplace/zzygx11m) of some correlationIds > 2**31, which was causing `ChromeTraceLogger::handleActivity` (in `output_json.cpp`) to skip writing the links here:
```
  if (op.flowId() > 0) {
    handleGenericLink(op);
  }
```

Reviewed By: aaronenyeshi, sraikund16

Differential Revision: D66827501

fbshipit-source-id: 1dd8748a4b81c0e9d703fbc80a450c5d350617e7

libkineto/include/GenericTraceActivity.h

@@ -75,7 +75,7 @@ class GenericTraceActivity : public ITraceActivity {
     return flow.type;
   }
 
-  int flowId() const override {
+  int64_t flowId() const override {
     return flow.id;
   }
 

libkineto/include/ITraceActivity.h

@@ -35,7 +35,7 @@ struct ITraceActivity {
   virtual int64_t correlationId() const = 0;
   // Part of a flow, identified by flow id and type
   virtual int flowType() const = 0;
-  virtual int flowId() const = 0;
+  virtual int64_t flowId() const = 0;
   virtual bool flowStart() const = 0;
   virtual ActivityType type() const = 0;
   virtual const std::string name() const = 0;

libkineto/src/CuptiActivity.h

@@ -80,7 +80,7 @@ struct CuptiActivity : public ITraceActivity {
   int flowType() const override {
     return kLinkAsyncCpuGpu;
   }
-  int flowId() const override {
+  int64_t flowId() const override {
     return correlationId();
   }
   const T& raw() const {

libkineto/src/RoctracerActivity.h

@@ -58,7 +58,7 @@ struct RoctracerActivity : public ITraceActivity {
   int flowType() const override {
     return kLinkAsyncCpuGpu;
   }
-  int flowId() const override {
+  int64_t flowId() const override {
     return correlationId();
   }
   const T& raw() const {