Remove unnecessary optionality

[scotts]

Because we set this value to 0 initially, it is always there. Since it's always there, there's no reason for it to be an optional. However, 0 is not a great default, since streams can technically have pts values less than 0. So let's just set it to be INT64_MIN, which is what we thought was appropriate if the value was not set (even though it always was!).

[scotts]

Benchmarks show this change is performance neutral. Three runs from before:

                                             |  10 seek()+next()  |  1 next()  |  10 next()  |  create()+next()
      TorchcodecCompiled                     |        48.9        |     9.8    |     13.4    |                 
      TorchcodecNonCompiled:                 |        48.9        |     9.6    |     12.9    |                 
      TorchcodecNonCompiled:num_threads=1    |       112.1        |     7.0    |     11.9    |                 
      TorchcodecNonCompiled                  |                    |            |             |        9.6   

      TorchcodecCompiled                     |        49.0        |     9.9    |     13.6    |                 
      TorchcodecNonCompiled:                 |        48.5        |     9.7    |     12.9    |                 
      TorchcodecNonCompiled:num_threads=1    |       113.1        |     7.0    |     12.0    |                 
      TorchcodecNonCompiled                  |                    |            |             |        9.7    

      TorchcodecCompiled                     |        49.1        |     9.8    |     13.5    |                 
      TorchcodecNonCompiled:                 |        49.2        |     9.6    |     14.4    |                 
      TorchcodecNonCompiled:num_threads=1    |       115.9        |     7.0    |     14.3    |                 
      TorchcodecNonCompiled                  |                    |            |             |        9.6            

After:

                                             |  10 seek()+next()  |  1 next()  |  10 next()  |  create()+next()
      TorchcodecCompiled                     |        48.7        |     9.8    |     13.5    |                 
      TorchcodecNonCompiled:                 |        49.1        |     9.5    |     14.7    |                 
      TorchcodecNonCompiled:num_threads=1    |       112.0        |     6.9    |     12.0    |                 
      TorchcodecNonCompiled                  |                    |            |             |        9.7      

      TorchcodecCompiled                     |        49.2        |     9.7    |     13.3    |                 
      TorchcodecNonCompiled:                 |        49.4        |     9.7    |     14.6    |                 
      TorchcodecNonCompiled:num_threads=1    |       112.0        |     7.0    |     12.1    |                 
      TorchcodecNonCompiled                  |                    |            |             |        9.6    

      TorchcodecCompiled                     |        49.0        |     9.7    |     13.4    |                 
      TorchcodecNonCompiled:                 |        48.2        |     9.6    |     12.8    |                 
      TorchcodecNonCompiled:num_threads=1    |       112.5        |     6.9    |     12.1    |                 
      TorchcodecNonCompiled                  |                    |            |             |        9.7      

[ahmadsharif1]

Is this comment stale now?

Alternative change would be to not initialize this to 0.

[scotts]

Yes, good catch. We can make it accurate with a one-word change.

Not initializing this variable to be 0 would actually have the same behavior as my change, although my change is simpler code. That is, we would use INT64_MIN until a seek happens in both cases.